这是让web2py的爱好者们高兴的三条新闻:

Best
Practices for Speeding Up Your Web Site (使用最优方法来加速运行你的网站)

本文转自:

  1. 流行的Python Web开发框架webpy获得InfoWorld开源大奖;
  2. web2py发布1.99.3版,正式向2.0版迈进;
  3. web2py book第四版发布。 流行的Python
    Web开发框架web2py发布了最新的1.99.3 stable版本。

今天看到这篇文,有空再翻译吧。先记录下来。

 

web2py刚刚获得了InfoWorld颁发的Bossie Awards
2011大奖,评奖编辑给予web2py的评价是“The
best open source application development software”。

  

The Exceptional Performance team has identified a number of best
practices for making web pages fast. The list includes 35 best practices
divided into 7 categories.

下载地址:
官方下载

80% of the end-user response time is spent on the front-end. Most of
this time is tied up in downloading all the components in the page:
images, stylesheets, scripts, Flash, etc. Reducing the number of
components in turn reduces the number of HTTP requests required to
render the page. This is the key to faster pages.

Filter by category:

为将要发布的web2py 2.0做准备,这个版本中进行了主要修订。
具体的更新如下:
moved to GitHub and abandoned Lanchpad
new web site layout, thanks Anthony
new welcome app using skeleton, thanks Anthony
jQuery 1.7.1
modernizr 2.0.6 (customized)
define_table(‘thing’, singluar=’thing’,plural=’things’)
CAS 1.0 and 2.0 compliant, thanks Olivier
fixed validate_and_insert and validate_and_update, thanks Anthony
request.user_agent().is_mobile etc., thanks Ross and Angelo
better router, thanks Jonathan
app on/off buttons
support for dropbox_login
improved markmin recognizes qr code, supports auto audio/video/embed
response.optimize_css = ‘concat,minify,inline’, thanks Ross
response.optimize_js = ‘concat,minify,inline’ thanks Ross
db.define_table(…,common_filter=query), thanks Yair
db(…,ignore_common_filter=True)
support for stripe payments
support for DowCommerce payments, thanks Dave
experimental PyPy support
experimental mongodb support, thanks Mark
tickets in db now accessible from admin, thanks Niphlod
many grid improvements and bug fixes

One way to reduce the number of components in the page is to simplify
the page’s design. But is there a way to build pages with richer content
while also achieving fast response times? Here are some techniques for
reducing the number of HTTP requests, while still supporting rich page
designs.

  • Content
  • Server
  • Cookie
  • CSS
  • JavaScript
  • Images
  • Mobile
  • All

和web2py 1.99.3同时发布的还有web2py
Book第四版,这是学习web2py开发的主要书籍,官方出品,以eBook形式发售,在线版免费。

Combined files are a way to reduce the number of HTTP requests by
combining all scripts into a single script, and similarly combining all
CSS into a single stylesheet. Combining files is more challenging when
the scripts and stylesheets vary from page to page, but making this part
of your release process improves response times.

Minimize HTTP Requests

tag: content

80% of the end-user response time is spent on the front-end. Most of
this time is tied up in downloading all the components in the page:
images, stylesheets, scripts, Flash, etc. Reducing the number of
components in turn reduces the number of HTTP requests required to
render the page. This is the key to faster pages.

One way to reduce the number of components in the page is to simplify
the page’s design. But is there a way to build pages with richer content
while also achieving fast response times? Here are some techniques for
reducing the number of HTTP requests, while still supporting rich page
designs.

Combined files are a way to reduce the number of HTTP requests by
combining all scripts into a single script, and similarly combining all
CSS into a single stylesheet. Combining files is more challenging when
the scripts and stylesheets vary from page to page, but making this part
of your release process improves response times.

CSS Sprites are the preferred method for reducing the number of
image requests. Combine your background images into a single image and
use the CSS background-image and background-position properties to
display the desired image segment.

Image maps combine multiple images into a single image. The overall
size is about the same, but reducing the number of HTTP requests speeds
up the page. Image maps only work if the images are contiguous in the
page, such as a navigation bar. Defining the coordinates of image maps
can be tedious and error prone. Using image maps for navigation is not
accessible too, so it’s not recommended.

Inline images use the data: URL scheme to embed the image data in
the actual page. This can increase the size of your HTML document.
Combining inline images into your (cached) stylesheets is a way to
reduce HTTP requests and avoid increasing the size of your pages. Inline
images are not yet supported across all major browsers.

Reducing the number of HTTP requests in your page is the place to start.
This is the most important guideline for improving performance for first
time visitors. As described in Tenni Theurer’s blog post Browser Cache
Usage – Exposed!, 40-60% of daily visitors to your site come in with an
empty cache. Making your page fast for these first time visitors is key
to a better user experience.

top | discuss this rule

(文/cnbeta)    

CSS Sprites are the
preferred method for reducing the number of image requests. Combine your
background images into a single image and use the CSS background-image
and background-position properties to display the desired image
segment.

Use a Content Delivery Network

tag: server

The user’s proximity to your web server has an impact on response times.
Deploying your content across multiple, geographically dispersed servers
will make your pages load faster from the user’s perspective. But where
should you start?

As a first step to implementing geographically dispersed content, don’t
attempt to redesign your web application to work in a distributed
architecture. Depending on the application, changing the architecture
could include daunting tasks such as synchronizing session state and
replicating database transactions across server locations. Attempts to
reduce the distance between users and your content could be delayed by,
or never pass, this application architecture step.

Remember that 80-90% of the end-user response time is spent downloading
all the components in the page: images, stylesheets, scripts, Flash,
etc. This is the Performance Golden Rule. Rather than starting with
the difficult task of redesigning your application architecture, it’s
better to first disperse your static content. This not only achieves a
bigger reduction in response times, but it’s easier thanks to content
delivery networks.

A content delivery network (CDN) is a collection of web servers
distributed across multiple locations to deliver content more
efficiently to users. The server selected for delivering content to a
specific user is typically based on a measure of network proximity. For
example, the server with the fewest network hops or the server with the
quickest response time is chosen.

Some large Internet companies own their own CDN, but it’s cost-effective
to use a CDN service provider, such as Akamai Technologies, EdgeCast, or
level3. For start-up companies and private web sites, the cost of a CDN
service can be prohibitive, but as your target audience grows larger and
becomes more global, a CDN is necessary to achieve fast response times.
At Yahoo!, properties that moved static content off their application
web servers to a CDN (both 3rd party as mentioned above as well as
Yahoo’s own CDN) improved end-user response times by 20% or more.
Switching to a CDN is a relatively easy code change that will
dramatically improve the speed of your web site.

top | discuss this rule

Image
maps
combine
multiple images into a single image. The overall size is about the same,
but reducing the number of HTTP requests speeds up the page. Image maps
only work if the images are contiguous in the page, such as a navigation
bar. Defining the coordinates of image maps can be tedious and error
prone. Using image maps for navigation is not accessible too, so it’s
not recommended.

Add an Expires or a Cache-Control Header

tag: server

There are two aspects to this rule:

  • For static components: implement “Never expire” policy by setting
    far future Expires header
  • For dynamic components: use an appropriate Cache-Control header to
    help the browser with conditional requests

 

Web page designs are getting richer and richer, which means more
scripts, stylesheets, images, and Flash in the page. A first-time
visitor to your page may have to make several HTTP requests, but by
using the Expires header you make those components cacheable. This
avoids unnecessary HTTP requests on subsequent page views. Expires
headers are most often used with images, but they should be used on
all components including scripts, stylesheets, and Flash components.

Browsers (and proxies) use a cache to reduce the number and size of HTTP
requests, making web pages load faster. A web server uses the Expires
header in the HTTP response to tell the client how long a component can
be cached. This is a far future Expires header, telling the browser that
this response won’t be stale until April 15, 2010.

      Expires: Thu, 15 Apr 2010 20:00:00 GMT

 

If your server is Apache, use the ExpiresDefault directive to set an
expiration date relative to the current date. This example of the
ExpiresDefault directive sets the Expires date 10 years out from the
time of the request.

      ExpiresDefault "access plus 10 years"

 

Keep in mind, if you use a far future Expires header you have to change
the component’s filename whenever the component changes. At Yahoo! we
often make this step part of the build process: a version number is
embedded in the component’s filename, for example, yahoo_2.0.6.js.

Using a far future Expires header affects page views only after a user
has already visited your site. It has no effect on the number of HTTP
requests when a user visits your site for the first time and the
browser’s cache is empty. Therefore the impact of this performance
improvement depends on how often users hit your pages with a primed
cache. (A “primed cache” already contains all of the components in the
page.) We measured this at Yahoo! and found the number of page views
with a primed cache is 75-85%. By using a far future Expires header, you
increase the number of components that are cached by the browser and
re-used on subsequent page views without sending a single byte over the
user’s Internet connection.

top | discuss this rule

Inline images use the data: URL
scheme to embed the image data in
the actual page. This can increase the size of your HTML document.
Combining inline images into your (cached) stylesheets is a way to
reduce HTTP requests and avoid increasing the size of your pages. Inline
images are not yet supported across all major browsers.

Gzip Components

tag: server

The time it takes to transfer an HTTP request and response across the
network can be significantly reduced by decisions made by front-end
engineers. It’s true that the end-user’s bandwidth speed, Internet
service provider, proximity to peering exchange points, etc. are beyond
the control of the development team. But there are other variables that
affect response times. Compression reduces response times by reducing
the size of the HTTP response.

Starting with HTTP/1.1, web clients indicate support for compression
with the Accept-Encoding header in the HTTP request.

      Accept-Encoding: gzip, deflate

 

If the web server sees this header in the request, it may compress the
response using one of the methods listed by the client. The web server
notifies the web client of this via the Content-Encoding header in the
response.

      Content-Encoding: gzip

 

Gzip is the most popular and effective compression method at this time.
It was developed by the GNU project and standardized by RFC 1952. The
only other compression format you’re likely to see is deflate, but it’s
less effective and less popular.

Gzipping generally reduces the response size by about 70%. Approximately
90% of today’s Internet traffic travels through browsers that claim to
support gzip. If you use Apache, the module configuring gzip depends on
your version: Apache 1.3 uses mod_gzip while Apache 2.x uses
mod_deflate.

There are known issues with browsers and proxies that may cause a
mismatch in what the browser expects and what it receives with regard to
compressed content. Fortunately, these edge cases are dwindling as the
use of older browsers drops off. The Apache modules help out by adding
appropriate Vary response headers automatically.

Servers choose what to gzip based on file type, but are typically too
limited in what they decide to compress. Most web sites gzip their HTML
documents. It’s also worthwhile to gzip your scripts and stylesheets,
but many web sites miss this opportunity. In fact, it’s worthwhile to
compress any text response including XML and JSON. Image and PDF files
should not be gzipped because they are already compressed. Trying to
gzip them not only wastes CPU but can potentially increase file sizes.

Gzipping as many file types as possible is an easy way to reduce page
weight and accelerate the user experience.

top | discuss this rule

Reducing the number of HTTP requests in your page is the place to start.
This is the most important guideline for improving performance for first
time visitors. As described in Tenni Theurer’s blog post Browser Cache
Usage –
Exposed!,
40-60% of daily visitors to your site come in with an empty cache.
Making your page fast for these first time visitors is key to a better
user experience.

Put Stylesheets at the Top

tag: css

While researching performance at Yahoo!, we discovered that moving
stylesheets to the document HEAD makes pages appear to be loading
faster. This is because putting stylesheets in the HEAD allows the page
to render progressively.

Front-end engineers that care about performance want a page to load
progressively; that is, we want the browser to display whatever content
it has as soon as possible. This is especially important for pages with
a lot of content and for users on slower Internet connections. The
importance of giving users visual feedback, such as progress indicators,
has been well researched and documented. In our case the HTML page is
the progress indicator! When the browser loads the page progressively
the header, the navigation bar, the logo at the top, etc. all serve as
visual feedback for the user who is waiting for the page. This improves
the overall user experience.

The problem with putting stylesheets near the bottom of the document is
that it prohibits progressive rendering in many browsers, including
Internet Explorer. These browsers block rendering to avoid having to
redraw elements of the page if their styles change. The user is stuck
viewing a blank white page.

The HTML specification clearly states that stylesheets are to be
included in the HEAD of the page: “Unlike A, [LINK] may only appear in
the HEAD section of a document, although it may appear any number of
times.” Neither of the alternatives, the blank white screen or flash of
unstyled content, are worth the risk. The optimal solution is to follow
the HTML specification and load your stylesheets in the document HEAD.

top | discuss this rule

top |
discuss this
rule

Put Scripts at the Bottom

tag: javascript

The problem caused by scripts is that they block parallel downloads. The
HTTP/1.1 specification suggests that browsers download no more than two
components in parallel per hostname. If you serve your images from
multiple hostnames, you can get more than two downloads to occur in
parallel. While a script is downloading, however, the browser won’t
start any other downloads, even on different hostnames.

In some situations it’s not easy to move scripts to the bottom. If, for
example, the script uses document.write to insert part of the page’s
content, it can’t be moved lower in the page. There might also be
scoping issues. In many cases, there are ways to workaround these
situations.

An alternative suggestion that often comes up is to use deferred
scripts. The DEFER attribute indicates that the script does not
contain document.write, and is a clue to browsers that they can continue
rendering. Unfortunately, Firefox doesn’t support the DEFER attribute.
In Internet Explorer, the script may be deferred, but not as much as
desired. If a script can be deferred, it can also be moved to the bottom
of the page. That will make your web pages load faster.

top | discuss this rule

Use a Content Delivery Network

tag: server

The user’s proximity to your web server has an impact on response times.
Deploying your content across multiple, geographically dispersed servers
will make your pages load faster from the user’s perspective. But where
should you start?

As a first step to implementing geographically dispersed content, don’t
attempt to redesign your web application to work in a distributed
architecture. Depending on the application, changing the architecture
could include daunting tasks such as synchronizing session state and
replicating database transactions across server locations. Attempts to
reduce the distance between users and your content could be delayed by,
or never pass, this application architecture step.

Remember that 80-90% of the end-user response time is spent downloading
all the components in the page: images, stylesheets, scripts, Flash,
etc. This is the Performance Golden Rule. Rather than starting with
the difficult task of redesigning your application architecture, it’s
better to first disperse your static content. This not only achieves a
bigger reduction in response times, but it’s easier thanks to content
delivery networks.

A content delivery network (CDN) is a collection of web servers
distributed across multiple locations to deliver content more
efficiently to users. The server selected for delivering content to a
specific user is typically based on a measure of network proximity. For
example, the server with the fewest network hops or the server with the
quickest response time is chosen.

Some large Internet companies own their own CDN, but it’s cost-effective
to use a CDN service provider, such as Akamai
Technologies, Mirror Image
Internet, or Limelight
Networks. For start-up companies and
private web sites, the cost of a CDN service can be prohibitive, but as
your target audience grows larger and becomes more global, a CDN is
necessary to achieve fast response times. At Yahoo!, properties that
moved static content off their application web servers to a CDN improved
end-user response times by 20% or more. Switching to a CDN is a
relatively easy code change that will dramatically improve the speed of
your web site.

top |
discuss this
rule

Avoid CSS Expressions

tag: css

CSS expressions are a powerful (and dangerous) way to set CSS properties
dynamically. They were supported in Internet Explorer starting with
version 5, but were deprecated starting with IE8. As an example, the
background color could be set to alternate every hour using CSS
expressions:

      background-color: expression( (new Date()).getHours()%2 ? "#B8D4FF" : "#F08A00" );

 

As shown here, the expression method accepts a JavaScript expression.
The CSS property is set to the result of evaluating the JavaScript
expression. The expression method is ignored by other browsers, so it
is useful for setting properties in Internet Explorer needed to create a
consistent experience across browsers.

The problem with expressions is that they are evaluated more frequently
than most people expect. Not only are they evaluated when the page is
rendered and resized, but also when the page is scrolled and even when
the user moves the mouse over the page. Adding a counter to the CSS
expression allows us to keep track of when and how often a CSS
expression is evaluated. Moving the mouse around the page can easily
generate more than 10,000 evaluations.

One way to reduce the number of times your CSS expression is evaluated
is to use one-time expressions, where the first time the expression is
evaluated it sets the style property to an explicit value, which
replaces the CSS expression. If the style property must be set
dynamically throughout the life of the page, using event handlers
instead of CSS expressions is an alternative approach. If you must use
CSS expressions, remember that they may be evaluated thousands of times
and could affect the performance of your page.

top | discuss this rule

Add an Expires or a Cache-Control Header

tag: server

There are two things in this rule:

  • For static components: implement “Never expire” policy by setting
    far future Expires header
  • For dynamic components: use an appropriate Cache-Control header to
    help the browser with conditional requests

Web page designs are getting richer and richer, which means more
scripts, stylesheets, images, and Flash in the page. A first-time
visitor to your page may have to make several HTTP requests, but by
using the Expires header you make those components cacheable. This
avoids unnecessary HTTP requests on subsequent page views. Expires
headers are most often used with images, but they should be used on
all components including scripts, stylesheets, and Flash components.

Browsers (and proxies) use a cache to reduce the number and size of HTTP
requests, making web pages load faster. A web server uses the Expires
header in the HTTP response to tell the client how long a component can
be cached. This is a far future Expires header, telling the browser that
this response won’t be stale until April 15, 2010.

 

      Expires: Thu, 15 Apr 2010 20:00:00 GMT

 

If your server is Apache, use the ExpiresDefault directive to set an
expiration date relative to the current date. This example of the
ExpiresDefault directive sets the Expires date 10 years out from the
time of the request.

 

      ExpiresDefault "access plus 10 years"

 

Keep in mind, if you use a far future Expires header you have to change
the component’s filename whenever the component changes. At Yahoo! we
often make this step part of the build process: a version number is
embedded in the component’s filename, for example, yahoo_2.0.6.js.

Using a far future Expires header affects page views only after a user
has already visited your site. It has no effect on the number of HTTP
requests when a user visits your site for the first time and the
browser’s cache is empty. Therefore the impact of this performance
improvement depends on how often users hit your pages with a primed
cache. (A “primed cache” already contains all of the components in the
page.) We measured this at
Yahoo!
and found the number of page views with a primed cache is 75-85%. By
using a far future Expires header, you increase the number of components
that are cached by the browser and re-used on subsequent page views
without sending a single byte over the user’s Internet connection.

top |
discuss this
rule

Make JavaScript and CSS External

tag: javascript, css

Many of these performance rules deal with how external components are
managed. However, before these considerations arise you should ask a
more basic question: Should JavaScript and CSS be contained in external
files, or inlined in the page itself?

Using external files in the real world generally produces faster pages
because the JavaScript and CSS files are cached by the browser.
JavaScript and CSS that are inlined in HTML documents get downloaded
every time the HTML document is requested. This reduces the number of
HTTP requests that are needed, but increases the size of the HTML
document. On the other hand, if the JavaScript and CSS are in external
files cached by the browser, the size of the HTML document is reduced
without increasing the number of HTTP requests.

The key factor, then, is the frequency with which external JavaScript
and CSS components are cached relative to the number of HTML documents
requested. This factor, although difficult to quantify, can be gauged
using various metrics. If users on your site have multiple page views
per session and many of your pages re-use the same scripts and
stylesheets, there is a greater potential benefit from cached external
files.

Many web sites fall in the middle of these metrics. For these sites, the
best solution generally is to deploy the JavaScript and CSS as external
files. The only exception where inlining is preferable is with home
pages, such as Yahoo!’s front page and My Yahoo!. Home pages that have
few (perhaps only one) page view per session may find that inlining
JavaScript and CSS results in faster end-user response times.

For front pages that are typically the first of many page views, there
are techniques that leverage the reduction of HTTP requests that
inlining provides, as well as the caching benefits achieved through
using external files. One such technique is to inline JavaScript and CSS
in the front page, but dynamically download the external files after the
page has finished loading. Subsequent pages would reference the external
files that should already be in the browser’s cache.

top | discuss this rule

Gzip Components

tag: server

The time it takes to transfer an HTTP request and response across the
network can be significantly reduced by decisions made by front-end
engineers. It’s true that the end-user’s bandwidth speed, Internet
service provider, proximity to peering exchange points, etc. are beyond
the control of the development team. But there are other variables that
affect response times. Compression reduces response times by reducing
the size of the HTTP response.

Starting with HTTP/1.1, web clients indicate support for compression
with the Accept-Encoding header in the HTTP request.

      Accept-Encoding: gzip, deflate

If the web server sees this header in the request, it may compress the
response using one of the methods listed by the client. The web server
notifies the web client of this via the Content-Encoding header in the
response.

      Content-Encoding: gzip

Gzip is the most popular and effective compression method at this time.
It was developed by the GNU project and standardized by RFC
1952. The only other compression
format you’re likely to see is deflate, but it’s less effective and less
popular.

Gzipping generally reduces the response size by about 70%. Approximately
90% of today’s Internet traffic travels through browsers that claim to
support gzip. If you use Apache, the module configuring gzip depends on
your version: Apache 1.3 uses
mod_gzip while Apache 2.x
uses
mod_deflate.

There are known issues with browsers and proxies that may cause a
mismatch in what the browser expects and what it receives with regard to
compressed content. Fortunately, these edge cases are dwindling as the
use of older browsers drops off. The Apache modules help out by adding
appropriate Vary response headers automatically.

Servers choose what to gzip based on file type, but are typically too
limited in what they decide to compress. Most web sites gzip their HTML
documents. It’s also worthwhile to gzip your scripts and stylesheets,
but many web sites miss this opportunity. In fact, it’s worthwhile to
compress any text response including XML and JSON. Image and PDF files
should not be gzipped because they are already compressed. Trying to
gzip them not only wastes CPU but can potentially increase file sizes.

Gzipping as many file types as possible is an easy way to reduce page
weight and accelerate the user experience.

top |
discuss this
rule

Reduce DNS Lookups

tag: content

The Domain Name System (DNS) maps hostnames to IP addresses, just as
phonebooks map people’s names to their phone numbers. When you type
www.yahoo.com into your browser, a DNS resolver contacted by the browser
returns that server’s IP address. DNS has a cost. It typically takes
20-120 milliseconds for DNS to lookup the IP address for a given
hostname. The browser can’t download anything from this hostname until
the DNS lookup is completed.

DNS lookups are cached for better performance. This caching can occur on
a special caching server, maintained by the user’s ISP or local area
network, but there is also caching that occurs on the individual user’s
computer. The DNS information remains in the operating system’s DNS
cache (the “DNS Client service” on Microsoft Windows). Most browsers
have their own caches, separate from the operating system’s cache. As
long as the browser keeps a DNS record in its own cache, it doesn’t
bother the operating system with a request for the record.

Internet Explorer caches DNS lookups for 30 minutes by default, as
specified by the DnsCacheTimeout registry setting. Firefox caches DNS
lookups for 1 minute, controlled by the network.dnsCacheExpiration
configuration setting. (Fasterfox changes this to 1 hour.)

When the client’s DNS cache is empty (for both the browser and the
operating system), the number of DNS lookups is equal to the number of
unique hostnames in the web page. This includes the hostnames used in
the page’s URL, images, script files, stylesheets, Flash objects, etc.
Reducing the number of unique hostnames reduces the number of DNS
lookups.

Reducing the number of unique hostnames has the potential to reduce the
amount of parallel downloading that takes place in the page. Avoiding
DNS lookups cuts response times, but reducing parallel downloads may
increase response times. My guideline is to split these components
across at least two but no more than four hostnames. This results in a
good compromise between reducing DNS lookups and allowing a high degree
of parallel downloads.

top | discuss this rule

Put Stylesheets at the Top

tag: css

While researching performance at Yahoo!, we discovered that moving
stylesheets to the document HEAD makes pages appear to be loading
faster. This is because putting stylesheets in the HEAD allows the page
to render progressively.

Front-end engineers that care about performance want a page to load
progressively; that is, we want the browser to display whatever content
it has as soon as possible. This is especially important for pages with
a lot of content and for users on slower Internet connections. The
importance of giving users visual feedback, such as progress indicators,
has been well researched and
documented. In our case
the HTML page is the progress indicator! When the browser loads the page
progressively the header, the navigation bar, the logo at the top, etc.
all serve as visual feedback for the user who is waiting for the page.
This improves the overall user experience.

The problem with putting stylesheets near the bottom of the document is
that it prohibits progressive rendering in many browsers, including
Internet Explorer. These browsers block rendering to avoid having to
redraw elements of the page if their styles change. The user is stuck
viewing a blank white page.

The HTML
specification
clearly states that stylesheets are to be included in the HEAD of the
page: “Unlike A, [LINK] may only appear in the HEAD section of a
document, although it may appear any number of times.” Neither of the
alternatives, the blank white screen or flash of unstyled content, are
worth the risk. The optimal solution is to follow the HTML specification
and load your stylesheets in the document HEAD.

top |
discuss this
rule

Minify JavaScript and CSS

tag: javascript, css

Minification is the practice of removing unnecessary characters from
code to reduce its size thereby improving load times. When code is
minified all comments are removed, as well as unneeded white space
characters (space, newline, and tab). In the case of JavaScript, this
improves response time performance because the size of the downloaded
file is reduced. Two popular tools for minifying JavaScript code are
JSMin and YUI Compressor. The YUI compressor can also minify CSS.

Obfuscation is an alternative optimization that can be applied to source
code. It’s more complex than minification and thus more likely to
generate bugs as a result of the obfuscation step itself. In a survey of
ten top U.S. web sites, minification achieved a 21% size reduction
versus 25% for obfuscation. Although obfuscation has a higher size
reduction, minifying JavaScript is less risky.

In addition to minifying external scripts and styles, inlined <script>
and <style> blocks can and should also be minified. Even if you gzip
your scripts and styles, minifying them will still reduce the size by 5%
or more. As the use and size of JavaScript and CSS increases, so will
the savings gained by minifying your code.

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Put Scripts at the Bottom

tag: javascript

The problem caused by scripts is that they block parallel downloads. The
HTTP/1.1
specification
suggests that browsers download no more than two components in parallel
per hostname. If you serve your images from multiple hostnames, you can
get more than two downloads to occur in parallel. While a script is
downloading, however, the browser won’t start any other downloads, even
on different hostnames.

In some situations it’s not easy to move scripts to the bottom. If, for
example, the script uses document.write to insert part of the page’s
content, it can’t be moved lower in the page. There might also be
scoping issues. In many cases, there are ways to workaround these
situations.

An alternative suggestion that often comes up is to use deferred
scripts. The DEFER attribute indicates that the script does not
contain document.write, and is a clue to browsers that they can continue
rendering. Unfortunately, Firefox doesn’t support the DEFER attribute.
In Internet Explorer, the script may be deferred, but not as much as
desired. If a script can be deferred, it can also be moved to the bottom
of the page. That will make your web pages load faster.

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rule

Avoid Redirects

tag: content

Redirects are accomplished using the 301 and 302 status codes. Here’s an
example of the HTTP headers in a 301 response:

      HTTP/1.1 301 Moved Permanently
      Location: http://example.com/newuri
      Content-Type: text/html

 

The browser automatically takes the user to the URL specified in the
Location field. All the information necessary for a redirect is in the
headers. The body of the response is typically empty. Despite their
names, neither a 301 nor a 302 response is cached in practice unless
additional headers, such as Expires or Cache-Control, indicate it
should be. The meta refresh tag and JavaScript are other ways to direct
users to a different URL, but if you must do a redirect, the preferred
technique is to use the standard 3xx HTTP status codes, primarily to
ensure the back button works correctly.

The main thing to remember is that redirects slow down the user
experience. Inserting a redirect between the user and the HTML document
delays everything in the page since nothing in the page can be rendered
and no components can start being downloaded until the HTML document has
arrived.

One of the most wasteful redirects happens frequently and web developers
are generally not aware of it. It occurs when a trailing slash (/) is
missing from a URL that should otherwise have one. For example, going to
results in a 301 response
containing a redirect to (notice
the added trailing slash). This is fixed in Apache by using Alias or
mod_rewrite, or the DirectorySlash directive if you’re using Apache
handlers.

Connecting an old web site to a new one is another common use for
redirects. Others include connecting different parts of a website and
directing the user based on certain conditions (type of browser, type of
user account, etc.). Using a redirect to connect two web sites is simple
and requires little additional coding. Although using redirects in these
situations reduces the complexity for developers, it degrades the user
experience. Alternatives for this use of redirects include using Alias
and mod_rewrite if the two code paths are hosted on the same server.
If a domain name change is the cause of using redirects, an alternative
is to create a CNAME (a DNS record that creates an alias pointing from
one domain name to another) in combination with Alias or
mod_rewrite.

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Avoid CSS Expressions

tag: css

CSS expressions are a powerful (and dangerous) way to set CSS properties
dynamically. They’re supported in Internet Explorer, starting with
version
5.
As an example, the background color could be set to alternate every hour
using CSS expressions.

 

      background-color: expression( (new Date()).getHours()%2 ? "#B8D4FF" : "#F08A00" );

 

As shown here, the expression method accepts a JavaScript expression.
The CSS property is set to the result of evaluating the JavaScript
expression. The expression method is ignored by other browsers, so it
is useful for setting properties in Internet Explorer needed to create a
consistent experience across browsers.

The problem with expressions is that they are evaluated more frequently
than most people expect. Not only are they evaluated when the page is
rendered and resized, but also when the page is scrolled and even when
the user moves the mouse over the page. Adding a counter to the CSS
expression allows us to keep track of when and how often a CSS
expression is evaluated. Moving the mouse around the page can easily
generate more than 10,000 evaluations.

One way to reduce the number of times your CSS expression is evaluated
is to use one-time expressions, where the first time the expression is
evaluated it sets the style property to an explicit value, which
replaces the CSS expression. If the style property must be set
dynamically throughout the life of the page, using event handlers
instead of CSS expressions is an alternative approach. If you must use
CSS expressions, remember that they may be evaluated thousands of times
and could affect the performance of your page.

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Remove Duplicate Scripts

tag: javascript

It hurts performance to include the same JavaScript file twice in one
page. This isn’t as unusual as you might think. A review of the ten top
U.S. web sites shows that two of them contain a duplicated script. Two
main factors increase the odds of a script being duplicated in a single
web page: team size and number of scripts. When it does happen,
duplicate scripts hurt performance by creating unnecessary HTTP requests
and wasted JavaScript execution.

Unnecessary HTTP requests happen in Internet Explorer, but not in
Firefox. In Internet Explorer, if an external script is included twice
and is not cacheable, it generates two HTTP requests during page
loading. Even if the script is cacheable, extra HTTP requests occur when
the user reloads the page.

In addition to generating wasteful HTTP requests, time is wasted
evaluating the script multiple times. This redundant JavaScript
execution happens in both Firefox and Internet Explorer, regardless of
whether the script is cacheable.

One way to avoid accidentally including the same script twice is to
implement a script management module in your templating system. The
typical way to include a script is to use the SCRIPT tag in your HTML
page.

      <script type="text/javascript" src="menu_1.0.17.js"></script>

 

An alternative in PHP would be to create a function called
insertScript.

      <?php insertScript("menu.js") ?>

 

In addition to preventing the same script from being inserted multiple
times, this function could handle other issues with scripts, such as
dependency checking and adding version numbers to script filenames to
support far future Expires headers.

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Make JavaScript and CSS External

tag: javascript, css

Many of these performance rules deal with how external components are
managed. However, before these considerations arise you should ask a
more basic question: Should JavaScript and CSS be contained in external
files, or inlined in the page itself?

Using external files in the real world generally produces faster pages
because the JavaScript and CSS files are cached by the browser.
JavaScript and CSS that are inlined in HTML documents get downloaded
every time the HTML document is requested. This reduces the number of
HTTP requests that are needed, but increases the size of the HTML
document. On the other hand, if the JavaScript and CSS are in external
files cached by the browser, the size of the HTML document is reduced
without increasing the number of HTTP requests.

The key factor, then, is the frequency with which external JavaScript
and CSS components are cached relative to the number of HTML documents
requested. This factor, although difficult to quantify, can be gauged
using various metrics. If users on your site have multiple page views
per session and many of your pages re-use the same scripts and
stylesheets, there is a greater potential benefit from cached external
files.

Many web sites fall in the middle of these metrics. For these sites, the
best solution generally is to deploy the JavaScript and CSS as external
files. The only exception where inlining is preferable is with home
pages, such as Yahoo!’s front page and My
Yahoo!. Home pages that have few (perhaps only
one) page view per session may find that inlining JavaScript and CSS
results in faster end-user response times.

For front pages that are typically the first of many page views, there
are techniques that leverage the reduction of HTTP requests that
inlining provides, as well as the caching benefits achieved through
using external files. One such technique is to inline JavaScript and CSS
in the front page, but dynamically download the external files after the
page has finished loading. Subsequent pages would reference the external
files that should already be in the browser’s cache.

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Configure ETags

tag: server

Entity tags (ETags) are a mechanism that web servers and browsers use to
determine whether the component in the browser’s cache matches the one
on the origin server. (An “entity” is another word a “component”:
images, scripts, stylesheets, etc.) ETags were added to provide a
mechanism for validating entities that is more flexible than the
last-modified date. An ETag is a string that uniquely identifies a
specific version of a component. The only format constraints are that
the string be quoted. The origin server specifies the component’s ETag
using the ETag response header.

      HTTP/1.1 200 OK
      Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT
      ETag: "10c24bc-4ab-457e1c1f"
      Content-Length: 12195

 

Later, if the browser has to validate a component, it uses the
If-None-Match header to pass the ETag back to the origin server. If
the ETags match, a 304 status code is returned reducing the response by
12195 bytes for this example.

      GET /i/yahoo.gif HTTP/1.1
      Host: us.yimg.com
      If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT
      If-None-Match: "10c24bc-4ab-457e1c1f"
      HTTP/1.1 304 Not Modified

 

The problem with ETags is that they typically are constructed using
attributes that make them unique to a specific server hosting a site.
ETags won’t match when a browser gets the original component from one
server and later tries to validate that component on a different server,
a situation that is all too common on Web sites that use a cluster of
servers to handle requests. By default, both Apache and IIS embed data
in the ETag that dramatically reduces the odds of the validity test
succeeding on web sites with multiple servers.

The ETag format for Apache 1.3 and 2.x is inode-size-timestamp.
Although a given file may reside in the same directory across multiple
servers, and have the same file size, permissions, timestamp, etc., its
inode is different from one server to the next.

IIS 5.0 and 6.0 have a similar issue with ETags. The format for ETags on
IIS is Filetimestamp:ChangeNumber. A ChangeNumber is a counter used
to track configuration changes to IIS. It’s unlikely that the
ChangeNumber is the same across all IIS servers behind a web site.

The end result is ETags generated by Apache and IIS for the exact same
component won’t match from one server to another. If the ETags don’t
match, the user doesn’t receive the small, fast 304 response that ETags
were designed for; instead, they’ll get a normal 200 response along with
all the data for the component. If you host your web site on just one
server, this isn’t a problem. But if you have multiple servers hosting
your web site, and you’re using Apache or IIS with the default ETag
configuration, your users are getting slower pages, your servers have a
higher load, you’re consuming greater bandwidth, and proxies aren’t
caching your content efficiently. Even if your components have a far
future Expires header, a conditional GET request is still made
whenever the user hits Reload or Refresh.

If you’re not taking advantage of the flexible validation model that
ETags provide, it’s better to just remove the ETag altogether. The
Last-Modified header validates based on the component’s timestamp. And
removing the ETag reduces the size of the HTTP headers in both the
response and subsequent requests. This Microsoft Support article
describes how to remove ETags. In Apache, this is done by simply adding
the following line to your Apache configuration file:

      FileETag none

 

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Reduce DNS Lookups

tag: content

The Domain Name System (DNS) maps hostnames to IP addresses, just as
phonebooks map people’s names to their phone numbers. When you type
www.yahoo.com into your browser, a DNS resolver contacted by the browser
returns that server’s IP address. DNS has a cost. It typically takes
20-120 milliseconds for DNS to lookup the IP address for a given
hostname. The browser can’t download anything from this hostname until
the DNS lookup is completed.

DNS lookups are cached for better performance. This caching can occur on
a special caching server, maintained by the user’s ISP or local area
network, but there is also caching that occurs on the individual user’s
computer. The DNS information remains in the operating system’s DNS
cache (the “DNS Client service” on Microsoft Windows). Most browsers
have their own caches, separate from the operating system’s cache. As
long as the browser keeps a DNS record in its own cache, it doesn’t
bother the operating system with a request for the record.

Internet Explorer caches DNS lookups for 30 minutes by default, as
specified by the DnsCacheTimeout registry setting. Firefox caches DNS
lookups for 1 minute, controlled by the network.dnsCacheExpiration
configuration setting. (Fasterfox changes this to 1 hour.)

When the client’s DNS cache is empty (for both the browser and the
operating system), the number of DNS lookups is equal to the number of
unique hostnames in the web page. This includes the hostnames used in
the page’s URL, images, script files, stylesheets, Flash objects, etc.
Reducing the number of unique hostnames reduces the number of DNS
lookups.

Reducing the number of unique hostnames has the potential to reduce the
amount of parallel downloading that takes place in the page. Avoiding
DNS lookups cuts response times, but reducing parallel downloads may
increase response times. My guideline is to split these components
across at least two but no more than four hostnames. This results in a
good compromise between reducing DNS lookups and allowing a high degree
of parallel downloads.

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rule

Make Ajax Cacheable

tag: content

One of the cited benefits of Ajax is that it provides instantaneous
feedback to the user because it requests information asynchronously from
the backend web server. However, using Ajax is no guarantee that the
user won’t be twiddling his thumbs waiting for those asynchronous
JavaScript and XML responses to return. In many applications, whether or
not the user is kept waiting depends on how Ajax is used. For example,
in a web-based email client the user will be kept waiting for the
results of an Ajax request to find all the email messages that match
their search criteria. It’s important to remember that “asynchronous”
does not imply “instantaneous”.

To improve performance, it’s important to optimize these Ajax responses.
The most important way to improve the performance of Ajax is to make the
responses cacheable, as discussed in Add an Expires or a Cache-Control
Header. Some of the other rules also apply to Ajax:

  • Gzip Components
  • Reduce DNS Lookups
  • Minify JavaScript
  • Avoid Redirects
  • Configure ETags

 

Let’s look at an example. A Web 2.0 email client might use Ajax to
download the user’s address book for autocompletion. If the user hasn’t
modified her address book since the last time she used the email web
app, the previous address book response could be read from cache if that
Ajax response was made cacheable with a future Expires or Cache-Control
header. The browser must be informed when to use a previously cached
address book response versus requesting a new one. This could be done by
adding a timestamp to the address book Ajax URL indicating the last time
the user modified her address book, for example, &t=1190241612. If the
address book hasn’t been modified since the last download, the timestamp
will be the same and the address book will be read from the browser’s
cache eliminating an extra HTTP roundtrip. If the user has modified her
address book, the timestamp ensures the new URL doesn’t match the cached
response, and the browser will request the updated address book entries.

Even though your Ajax responses are created dynamically, and might only
be applicable to a single user, they can still be cached. Doing so will
make your Web 2.0 apps faster.

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Minify JavaScript and CSS

tag: javascript, css

Minification is the practice of removing unnecessary characters from
code to reduce its size thereby improving load times. When code is
minified all comments are removed, as well as unneeded white space
characters (space, newline, and tab). In the case of JavaScript, this
improves response time performance because the size of the downloaded
file is reduced. Two popular tools for minifying JavaScript code are
JSMin and YUI
Compressor. The YUI
compressor can also minify CSS.

Obfuscation is an alternative optimization that can be applied to source
code. It’s more complex than minification and thus more likely to
generate bugs as a result of the obfuscation step itself. In a survey of
ten top U.S. web sites, minification achieved a 21% size reduction
versus 25% for obfuscation. Although obfuscation has a higher size
reduction, minifying JavaScript is less risky.

In addition to minifying external scripts and styles, inlined <script>
and <style> blocks can and should also be minified. Even if you gzip
your scripts and styles, minifying them will still reduce the size by 5%
or more. As the use and size of JavaScript and CSS increases, so will
the savings gained by minifying your code.

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rule

Flush the Buffer Early

tag: server

When users request a page, it can take anywhere from 200 to 500ms for
the backend server to stitch together the HTML page. During this time,
the browser is idle as it waits for the data to arrive. In PHP you have
the function flush(). It allows you to send your partially ready HTML
response to the browser so that the browser can start fetching
components while your backend is busy with the rest of the HTML page.
The benefit is mainly seen on busy backends or light frontends.

A good place to consider flushing is right after the HEAD because the
HTML for the head is usually easier to produce and it allows you to
include any CSS and JavaScript files for the browser to start fetching
in parallel while the backend is still processing.

Example:

      ... <!-- css, js -->
    </head>
    <?php flush(); ?>
    <body>
      ... <!-- content -->

 

Yahoo! search pioneered research and real user testing to prove the
benefits of using this technique.

top

Avoid Redirects

tag: content

Redirects are accomplished using the 301 and 302 status codes. Here’s an
example of the HTTP headers in a 301 response:

 

      HTTP/1.1 301 Moved Permanently
Location: http://example.com/newuri
Content-Type: text/html

 

The browser automatically takes the user to the URL specified in the
Location field. All the information necessary for a redirect is in the
headers. The body of the response is typically empty. Despite their
names, neither a 301 nor a 302 response is cached in practice unless
additional headers, such as Expires or Cache-Control, indicate it
should be. The meta refresh tag and JavaScript are other ways to direct
users to a different URL, but if you must do a redirect, the preferred
technique is to use the standard 3xx HTTP status codes, primarily to
ensure the back button works correctly.

The main thing to remember is that redirects slow down the user
experience. Inserting a redirect between the user and the HTML document
delays everything in the page since nothing in the page can be rendered
and no components can start being downloaded until the HTML document has
arrived.

One of the most wasteful redirects happens frequently and web developers
are generally not aware of it. It occurs when a trailing slash (/) is
missing from a URL that should otherwise have one. For example, going to
results in a 301 response
containing a redirect to (notice
the added trailing slash). This is fixed in Apache by using Alias or
mod_rewrite, or the DirectorySlash directive if you’re using Apache
handlers.

Connecting an old web site to a new one is another common use for
redirects. Others include connecting different parts of a website and
directing the user based on certain conditions (type of browser, type of
user account, etc.). Using a redirect to connect two web sites is simple
and requires little additional coding. Although using redirects in these
situations reduces the complexity for developers, it degrades the user
experience. Alternatives for this use of redirects include using Alias
and mod_rewrite if the two code paths are hosted on the same server.
If a domain name change is the cause of using redirects, an alternative
is to create a CNAME (a DNS record that creates an alias pointing from
one domain name to another) in combination with Alias or
mod_rewrite.

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rule

Use GET for AJAX Requests

tag: server

The Yahoo! Mail team found that when using XMLHttpRequest, POST is
implemented in the browsers as a two-step process: sending the headers
first, then sending data. So it’s best to use GET, which only takes one
TCP packet to send (unless you have a lot of cookies). The maximum URL
length in IE is 2K, so if you send more than 2K data you might not be
able to use GET.

An interesting side affect is that POST without actually posting any
data behaves like GET. Based on the HTTP specs, GET is meant for
retrieving information, so it makes sense (semantically) to use GET when
you’re only requesting data, as opposed to sending data to be stored
server-side.

 

top

Remove Duplicate Scripts

tag: javascript

It hurts performance to include the same JavaScript file twice in one
page. This isn’t as unusual as you might think. A review of the ten top
U.S. web sites shows that two of them contain a duplicated script. Two
main factors increase the odds of a script being duplicated in a single
web page: team size and number of scripts. When it does happen,
duplicate scripts hurt performance by creating unnecessary HTTP requests
and wasted JavaScript execution.

Unnecessary HTTP requests happen in Internet Explorer, but not in
Firefox. In Internet Explorer, if an external script is included twice
and is not cacheable, it generates two HTTP requests during page
loading. Even if the script is cacheable, extra HTTP requests occur when
the user reloads the page.

In addition to generating wasteful HTTP requests, time is wasted
evaluating the script multiple times. This redundant JavaScript
execution happens in both Firefox and Internet Explorer, regardless of
whether the script is cacheable.

One way to avoid accidentally including the same script twice is to
implement a script management module in your templating system. The
typical way to include a script is to use the SCRIPT tag in your HTML
page.

 

      <script type="text/javascript" src="menu_1.0.17.js"></script>

An alternative in PHP would be to create a function called
insertScript.

 

      <?php insertScript("menu.js") ?>

In addition to preventing the same script from being inserted multiple
times, this function could handle other issues with scripts, such as
dependency checking and adding version numbers to script filenames to
support far future Expires headers.

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rule

Post-load Components

tag: content

You can take a closer look at your page and ask yourself: “What’s
absolutely required in order to render the page initially?”. The rest of
the content and components can wait.

JavaScript is an ideal candidate for splitting before and after the
onload event. For example if you have JavaScript code and libraries that
do drag and drop and animations, those can wait, because dragging
elements on the page comes after the initial rendering. Other places to
look for candidates for post-loading include hidden content (content
that appears after a user action) and images below the fold.

Tools to help you out in your effort: YUI Image Loader allows you to
delay images below the fold and the YUI Get utility is an easy way to
include JS and CSS on the fly. For an example in the wild take a look at
Yahoo! Home Page with Firebug’s Net Panel turned on.

It’s good when the performance goals are inline with other web
development best practices. In this case, the idea of progressive
enhancement tells us that JavaScript, when supported, can improve the
user experience but you have to make sure the page works even without
JavaScript. So after you’ve made sure the page works fine, you can
enhance it with some post-loaded scripts that give you more bells and
whistles such as drag and drop and animations.

top

Configure ETags

tag: server

Entity tags (ETags) are a mechanism that web servers and browsers use to
determine whether the component in the browser’s cache matches the one
on the origin server. (An “entity” is another word a “component”:
images, scripts, stylesheets, etc.) ETags were added to provide a
mechanism for validating entities that is more flexible than the
last-modified date. An ETag is a string that uniquely identifies a
specific version of a component. The only format constraints are that
the string be quoted. The origin server specifies the component’s ETag
using the ETag response header.

 

      HTTP/1.1 200 OK
Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT
ETag: "10c24bc-4ab-457e1c1f"
Content-Length: 12195

 

Later, if the browser has to validate a component, it uses the
If-None-Match header to pass the ETag back to the origin server. If
the ETags match, a 304 status code is returned reducing the response by
12195 bytes for this example.

 

      GET /i/yahoo.gif HTTP/1.1
Host: us.yimg.com
If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT
If-None-Match: "10c24bc-4ab-457e1c1f"
HTTP/1.1 304 Not Modified

 

The problem with ETags is that they typically are constructed using
attributes that make them unique to a specific server hosting a site.
ETags won’t match when a browser gets the original component from one
server and later tries to validate that component on a different server,
a situation that is all too common on Web sites that use a cluster of
servers to handle requests. By default, both Apache and IIS embed data
in the ETag that dramatically reduces the odds of the validity test
succeeding on web sites with multiple servers.

The ETag format for Apache 1.3 and 2.x is inode-size-timestamp.
Although a given file may reside in the same directory across multiple
servers, and have the same file size, permissions, timestamp, etc., its
inode is different from one server to the next.

IIS 5.0 and 6.0 have a similar issue with ETags. The format for ETags on
IIS is Filetimestamp:ChangeNumber. A ChangeNumber is a counter used
to track configuration changes to IIS. It’s unlikely that the
ChangeNumber is the same across all IIS servers behind a web site.

The end result is ETags generated by Apache and IIS for the exact same
component won’t match from one server to another. If the ETags don’t
match, the user doesn’t receive the small, fast 304 response that ETags
were designed for; instead, they’ll get a normal 200 response along with
all the data for the component. If you host your web site on just one
server, this isn’t a problem. But if you have multiple servers hosting
your web site, and you’re using Apache or IIS with the default ETag
configuration, your users are getting slower pages, your servers have a
higher load, you’re consuming greater bandwidth, and proxies aren’t
caching your content efficiently. Even if your components have a far
future Expires header, a conditional GET request is still made
whenever the user hits Reload or Refresh.

If you’re not taking advantage of the flexible validation model that
ETags provide, it’s better to just remove the ETag altogether. The
Last-Modified header validates based on the component’s timestamp. And
removing the ETag reduces the size of the HTTP headers in both the
response and subsequent requests. This Microsoft Support
article describes how to
remove ETags. In Apache, this is done by simply adding the following
line to your Apache configuration file:

      FileETag none

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rule

Preload Components

tag: content

Preload may look like the opposite of post-load, but it actually has a
different goal. By preloading components you can take advantage of the
time the browser is idle and request components (like images, styles and
scripts) you’ll need in the future. This way when the user visits the
next page, you could have most of the components already in the cache
and your page will load much faster for the user.

There are actually several types of preloading:

  • Unconditional preload – as soon as onload fires, you go ahead and
    fetch some extra components. Check google.com for an example of how
    a sprite image is requested onload. This sprite image is not needed
    on the google.com homepage, but it is needed on the consecutive
    search result page.
  • Conditional preload – based on a user action you make an educated
    guess where the user is headed next and preload accordingly. On
    search.yahoo.com you can see how some extra components are requested
    after you start typing in the input box.
  • Anticipated preload – preload in advance before launching a
    redesign. It often happens after a redesign that you hear: “The new
    site is cool, but it’s slower than before”. Part of the problem
    could be that the users were visiting your old site with a full
    cache, but the new one is always an empty cache experience. You can
    mitigate this side effect by preloading some components before you
    even launched the redesign. Your old site can use the time the
    browser is idle and request images and scripts that will be used by
    the new site

top

Make Ajax Cacheable

tag: content

One of the cited benefits of Ajax is that it provides instantaneous
feedback to the user because it requests information asynchronously from
the backend web server. However, using Ajax is no guarantee that the
user won’t be twiddling his thumbs waiting for those asynchronous
JavaScript and XML responses to return. In many applications, whether or
not the user is kept waiting depends on how Ajax is used. For example,
in a web-based email client the user will be kept waiting for the
results of an Ajax request to find all the email messages that match
their search criteria. It’s important to remember that “asynchronous”
does not imply “instantaneous”.

To improve performance, it’s important to optimize these Ajax responses.
The most important way to improve the performance of Ajax is to make the
responses cacheable, as discussed in Add an Expires or a Cache-Control
Header. Some
of the other rules also apply to Ajax:

  • Gzip
    Components
  • Reduce DNS
    Lookups
  • Minify
    JavaScript
  • Avoid
    Redirects
  • Configure
    ETags

 

Let’s look at an example. A Web 2.0 email client might use Ajax to
download the user’s address book for autocompletion. If the user hasn’t
modified her address book since the last time she used the email web
app, the previous address book response could be read from cache if that
Ajax response was made cacheable with a future Expires or Cache-Control
header. The browser must be informed when to use a previously cached
address book response versus requesting a new one. This could be done by
adding a timestamp to the address book Ajax URL indicating the last time
the user modified her address book, for example, &t=1190241612. If the
address book hasn’t been modified since the last download, the timestamp
will be the same and the address book will be read from the browser’s
cache eliminating an extra HTTP roundtrip. If the user has modified her
address book, the timestamp ensures the new URL doesn’t match the cached
response, and the browser will request the updated address book entries.

Even though your Ajax responses are created dynamically, and might only
be applicable to a single user, they can still be cached. Doing so will
make your Web 2.0 apps faster.

top |
discuss this
rule

Reduce the Number of DOM Elements

tag: content

A complex page means more bytes to download and it also means slower DOM
access in JavaScript. It makes a difference if you loop through 500 or
5000 DOM elements on the page when you want to add an event handler for
example.

A high number of DOM elements can be a symptom that there’s something
that should be improved with the markup of the page without necessarily
removing content. Are you using nested tables for layout purposes? Are
you throwing in more <div>s only to fix layout issues? Maybe there’s a
better and more semantically correct way to do your markup.

A great help with layouts are the YUI CSS utilities: grids.css can help
you with the overall layout, fonts.css and reset.css can help you strip
away the browser’s defaults formatting. This is a chance to start fresh
and think about your markup, for example use <div>s only when it makes
sense semantically, and not because it renders a new line.

The number of DOM elements is easy to test, just type in Firebug’s
console:
document.getElementsByTagName('*').length

And how many DOM elements are too many? Check other similar pages that
have good markup. For example the Yahoo! Home Page is a pretty busy page
and still under 700 elements (HTML tags).

top

Flush the Buffer Early

tag: server

When users request a page, it can take anywhere from 200 to 500ms for
the backend server to stitch together the HTML page. During this time,
the browser is idle as it waits for the data to arrive. In PHP you have
the function flush(). It allows you to send your
partially ready HTML response to the browser so that the browser can
start fetching components while your backend is busy with the rest of
the HTML page. The benefit is mainly seen on busy backends or light
frontends.

A good place to consider flushing is right after the HEAD because the
HTML for the head is usually easier to produce and it allows you to
include any CSS and JavaScript files for the browser to start fetching
in parallel while the backend is still processing.

Example:

      ... <!-- css, js -->
</head>
<?php flush(); ?>
<body>
... <!-- content -->

Yahoo! search pioneered research and real
user testing to prove the benefits of using this technique.

top

Split Components Across Domains

tag: content

Splitting components allows you to maximize parallel downloads. Make
sure you’re using not more than 2-4 domains because of the DNS lookup
penalty. For example, you can host your HTML and dynamic content on
www.example.org and split static components between
static1.example.org and static2.example.org

For more information check “Maximizing Parallel Downloads in the Carpool
Lane” by Tenni Theurer and Patty Chi.

top

Use GET for AJAX Requests

tag: server

The Yahoo! Mail team found that when using
XMLHttpRequest, POST is implemented in the browsers as a two-step
process: sending the headers first, then sending data. So it’s best to
use GET, which only takes one TCP packet to send (unless you have a lot
of cookies). The maximum URL length in IE is 2K, so if you send more
than 2K data you might not be able to use GET.

An interesting side affect is that POST without actually posting any
data behaves like GET. Based on the HTTP
specs, GET is
meant for retrieving information, so it makes sense (semantically) to
use GET when you’re only requesting data, as opposed to sending data to
be stored server-side.

 

top

Minimize the Number of iframes

tag: content

Iframes allow an HTML document to be inserted in the parent document.
It’s important to understand how iframes work so they can be used
effectively.

<iframe> pros:

  • Helps with slow third-party content like badges and ads
  • Security sandbox
  • Download scripts in parallel

<iframe> cons:

  • Costly even if blank
  • Blocks page onload
  • Non-semantic

top

Post-load Components

tag: content

You can take a closer look at your page and ask yourself: “What’s
absolutely required in order to render the page initially?”. The rest of
the content and components can wait.

JavaScript is an ideal candidate for splitting before and after the
onload event. For example if you have JavaScript code and libraries that
do drag and drop and animations, those can wait, because dragging
elements on the page comes after the initial rendering. Other places to
look for candidates for post-loading include hidden content (content
that appears after a user action) and images below the fold.

Tools to help you out in your effort: YUI Image
Loader allows you to delay
images below the fold and the YUI Get
utility is an easy way to include
JS and CSS on the fly. For an example in the wild take a look at Yahoo!
Home Page with Firebug’s Net Panel turned on.

It’s good when the performance goals are inline with other web
development best practices. In this case, the idea of progressive
enhancement tells us that JavaScript, when supported, can improve the
user experience but you have to make sure the page works even without
JavaScript. So after you’ve made sure the page works fine, you can
enhance it with some post-loaded scripts that give you more bells and
whistles such as drag and drop and animations.

top

No 404s

tag: content

HTTP requests are expensive so making an HTTP request and getting a
useless response (i.e. 404 Not Found) is totally unnecessary and will
slow down the user experience without any benefit.

Some sites have helpful 404s “Did you mean X?”, which is great for the
user experience but also wastes server resources (like database, etc).
Particularly bad is when the link to an external JavaScript is wrong and
the result is a 404. First, this download will block parallel downloads.
Next the browser may try to parse the 404 response body as if it were
JavaScript code, trying to find something usable in it.

top

Preload Components

tag: content

Preload may look like the opposite of post-load, but it actually has a
different goal. By preloading components you can take advantage of the
time the browser is idle and request components (like images, styles and
scripts) you’ll need in the future. This way when the user visits the
next page, you could have most of the components already in the cache
and your page will load much faster for the user.

There are actually several types of preloading:

  • Unconditional preload – as soon as onload fires, you go ahead and
    fetch some extra components. Check google.com for an example of how
    a sprite image is requested onload. This sprite image is not needed
    on the google.com homepage, but it is needed on the consecutive
    search result page.
  • Conditional preload – based on a user action you make an educated
    guess where the user is headed next and preload accordingly. On
    search.yahoo.com you can see how some
    extra components are requested after you start typing in the input
    box.
  • Anticipated preload – preload in advance before launching a
    redesign. It often happens after a redesign that you hear: “The new
    site is cool, but it’s slower than before”. Part of the problem
    could be that the users were visiting your old site with a full
    cache, but the new one is always an empty cache experience. You can
    mitigate this side effect by preloading some components before you
    even launched the redesign. Your old site can use the time the
    browser is idle and request images and scripts that will be used by
    the new site

top

Reduce Cookie Size

tag: cookie

HTTP cookies are used for a variety of reasons such as authentication
and personalization. Information about cookies is exchanged in the HTTP
headers between web servers and browsers. It’s important to keep the
size of cookies as low as possible to minimize the impact on the user’s
response time.

For more information check “When the Cookie Crumbles” by Tenni Theurer
and Patty Chi. The take-home of this research:

  • Eliminate unnecessary cookies
  • Keep cookie sizes as low as possible to minimize the impact on the
    user response time
  • Be mindful of setting cookies at the appropriate domain level so
    other sub-domains are not affected
  • Set an Expires date appropriately. An earlier Expires date or none
    removes the cookie sooner, improving the user response time

top

Reduce the Number of DOM Elements

tag: content

A complex page means more bytes to download and it also means slower DOM
access in JavaScript. It makes a difference if you loop through 500 or
5000 DOM elements on the page when you want to add an event handler for
example.

A high number of DOM elements can be a symptom that there’s something
that should be improved with the markup of the page without necessarily
removing content. Are you using nested tables for layout purposes? Are
you throwing in more <div>s only to fix layout issues? Maybe there’s a
better and more semantically correct way to do your markup.

A great help with layouts are the YUI CSS
utilities: grids.css can help you with
the overall layout, fonts.css and reset.css can help you strip away the
browser’s defaults formatting. This is a chance to start fresh and think
about your markup, for example use <div>s only when it makes sense
semantically, and not because it renders a new line.

The number of DOM elements is easy to test, just type in Firebug’s
console:
document.getElementsByTagName('*').length

And how many DOM elements are too many? Check other similar pages that
have good markup. For example the Yahoo! Home
Page is a pretty busy page and still under 700
elements (HTML tags).

top

Use Cookie-free Domains for Components

tag: cookie

When the browser makes a request for a static image and sends cookies
together with the request, the server doesn’t have any use for those
cookies. So they only create network traffic for no good reason. You
should make sure static components are requested with cookie-free
requests. Create a subdomain and host all your static components there.

If your domain is www.example.org, you can host your static components
on static.example.org. However, if you’ve already set cookies on the
top-level domain example.org as opposed to www.example.org, then all
the requests to static.example.org will include those cookies. In this
case, you can buy a whole new domain, host your static components there,
and keep this domain cookie-free. Yahoo! uses yimg.com, YouTube uses
ytimg.com, Amazon uses images-amazon.com and so on.

Another benefit of hosting static components on a cookie-free domain is
that some proxies might refuse to cache the components that are
requested with cookies. On a related note, if you wonder if you should
use example.org or www.example.org for your home page, consider the
cookie impact. Omitting www leaves you no choice but to write cookies to
*.example.org, so for performance reasons it’s best to use the www
subdomain and write the cookies to that subdomain.

top

Split Components Across Domains

tag: content

Splitting components allows you to maximize parallel downloads. Make
sure you’re using not more than 2-4 domains because of the DNS lookup
penalty. For example, you can host your HTML and dynamic content on
www.example.org and split static components between
static1.example.org and static2.example.org

For more information check “Maximizing Parallel Downloads in the
Carpool
Lane”
by Tenni Theurer and Patty Chi.

top

Minimize DOM Access

tag: javascript

Accessing DOM elements with JavaScript is slow so in order to have a
more responsive page, you should:

  • Cache references to accessed elements
  • Update nodes “offline” and then add them to the tree
  • Avoid fixing layout with JavaScript

For more information check the YUI theatre’s “High Performance Ajax
Applications” by Julien Lecomte.

top

Minimize the Number of iframes

tag: content

Iframes allow an HTML document to be inserted in the parent document.
It’s important to understand how iframes work so they can be used
effectively.

<iframe> pros:

  • Helps with slow third-party content like badges and ads
  • Security sandbox
  • Download scripts in parallel

<iframe> cons:

  • Costly even if blank
  • Blocks page onload
  • Non-semantic

top

Develop Smart Event Handlers

tag: javascript

Sometimes pages feel less responsive because of too many event handlers
attached to different elements of the DOM tree which are then executed
too often. That’s why using event delegation is a good approach. If
you have 10 buttons inside a div, attach only one event handler to the
div wrapper, instead of one handler for each button. Events bubble up so
you’ll be able to catch the event and figure out which button it
originated from.

You also don’t need to wait for the onload event in order to start doing
something with the DOM tree. Often all you need is the element you want
to access to be available in the tree. You don’t have to wait for all
images to be downloaded. DOMContentLoaded is the event you might
consider using instead of onload, but until it’s available in all
browsers, you can use the YUI Event utility, which has an onAvailable
method.

For more information check the YUI theatre’s “High Performance Ajax
Applications” by Julien Lecomte.

top

No 404s

tag: content

HTTP requests are expensive so making an HTTP request and getting a
useless response (i.e. 404 Not Found) is totally unnecessary and will
slow down the user experience without any benefit.

Some sites have helpful 404s “Did you mean X?”, which is great for the
user experience but also wastes server resources (like database, etc).
Particularly bad is when the link to an external JavaScript is wrong and
the result is a 404. First, this download will block parallel downloads.
Next the browser may try to parse the 404 response body as if it were
JavaScript code, trying to find something usable in it.

top

Choose <link> over @import

tag: css

One of the previous best practices states that CSS should be at the top
in order to allow for progressive rendering.

In IE @import behaves the same as using <link> at the bottom of the
page, so it’s best not to use it.

top

Reduce Cookie Size

tag: cookie

HTTP cookies are used for a variety of reasons such as authentication
and personalization. Information about cookies is exchanged in the HTTP
headers between web servers and browsers. It’s important to keep the
size of cookies as low as possible to minimize the impact on the user’s
response time.

For more information check “When the Cookie
Crumbles”
by Tenni Theurer and Patty Chi. The take-home of this research:

  • Eliminate unnecessary cookies
  • Keep cookie sizes as low as possible to minimize the impact on the
    user response time
  • Be mindful of setting cookies at the appropriate domain level so
    other sub-domains are not affected
  • Set an Expires date appropriately. An earlier Expires date or none
    removes the cookie sooner, improving the user response time

top

Avoid Filters

tag: css

The IE-proprietary AlphaImageLoader filter aims to fix a problem with
semi-transparent true color PNGs in IE versions < 7. The problem with
this filter is that it blocks rendering and freezes the browser while
the image is being downloaded. It also increases memory consumption and
is applied per element, not per image, so the problem is multiplied.

The best approach is to avoid AlphaImageLoader completely and use
gracefully degrading PNG8 instead, which are fine in IE. If you
absolutely need AlphaImageLoader, use the underscore hack _filter as
to not penalize your IE7+ users.

top

Use Cookie-free Domains for Components

tag: cookie

When the browser makes a request for a static image and sends cookies
together with the request, the server doesn’t have any use for those
cookies. So they only create network traffic for no good reason. You
should make sure static components are requested with cookie-free
requests. Create a subdomain and host all your static components there.

If your domain is www.example.org, you can host your static components
on static.example.org. However, if you’ve already set cookies on the
top-level domain example.org as opposed to www.example.org, then all
the requests to static.example.org will include those cookies. In this
case, you can buy a whole new domain, host your static components there,
and keep this domain cookie-free. Yahoo! uses yimg.com, YouTube uses
ytimg.com, Amazon uses images-amazon.com and so on.

Another benefit of hosting static components on a cookie-free domain is
that some proxies might refuse to cache the components that are
requested with cookies. On a related note, if you wonder if you should
use example.org or www.example.org for your home page, consider the
cookie impact. Omitting www leaves you no choice but to write cookies to
*.example.org, so for performance reasons it’s best to use the www
subdomain and write the cookies to that subdomain.

top

Optimize Images

tag: images

After a designer is done with creating the images for your web page,
there are still some things you can try before you FTP those images to
your web server.

  • You can check the GIFs and see if they are using a palette size
    corresponding to the number of colors in the image. Using
    imagemagick it’s easy to check using
    identify -verbose image.gif
    When you see an image useing 4 colors and a 256 color “slots” in the
    palette, there is room for improvement.
  • Try converting GIFs to PNGs and see if there is a saving. More often
    than not, there is. Developers often hesitate to use PNGs due to the
    limited support in browsers, but this is now a thing of the past.
    The only real problem is alpha-transparency in true color PNGs, but
    then again, GIFs are not true color and don’t support variable
    transparency either. So anything a GIF can do, a palette PNG (PNG8)
    can do too (except for animations). This simple imagemagick command
    results in totally safe-to-use PNGs:
    convert image.gif image.png
    “All we are saying is: Give PiNG a Chance!”
  • Run pngcrush (or any other PNG optimizer tool) on all your PNGs.
    Example:
    pngcrush image.png -rem alla -reduce -brute result.png
  • Run jpegtran on all your JPEGs. This tool does lossless JPEG
    operations such as rotation and can also be used to optimize and
    remove comments and other useless information (such as EXIF
    information) from your images.
    jpegtran -copy none -optimize -perfect src.jpg dest.jpg

top

Minimize DOM Access

tag: javascript

Accessing DOM elements with JavaScript is slow so in order to have a
more responsive page, you should:

  • Cache references to accessed elements
  • Update nodes “offline” and then add them to the tree
  • Avoid fixing layout with JavaScript

For more information check the YUI theatre’s “High Performance Ajax
Applications” by
Julien Lecomte.

top

Optimize CSS Sprites

tag: images

  • Arranging the images in the sprite horizontally as opposed to
    vertically usually results in a smaller file size.
  • Combining similar colors in a sprite helps you keep the color count
    low, ideally under 256 colors so to fit in a PNG8.
  • “Be mobile-friendly” and don’t leave big gaps between the images in
    a sprite. This doesn’t affect the file size as much but requires
    less memory for the user agent to decompress the image into a pixel
    map. 100×100 image is 10 thousand pixels, where 1000×1000 is 1
    million pixels

top

Develop Smart Event Handlers

tag: javascript

Sometimes pages feel less responsive because of too many event handlers
attached to different elements of the DOM tree which are then executed
too often. That’s why using event delegation is a good approach. If
you have 10 buttons inside a div, attach only one event handler to the
div wrapper, instead of one handler for each button. Events bubble up so
you’ll be able to catch the event and figure out which button it
originated from.

You also don’t need to wait for the onload event in order to start doing
something with the DOM tree. Often all you need is the element you want
to access to be available in the tree. You don’t have to wait for all
images to be downloaded. DOMContentLoaded is the event you might
consider using instead of onload, but until it’s available in all
browsers, you can use the YUI
Event utility, which has an
onAvailable method.

For more information check the YUI theatre’s “High Performance Ajax
Applications” by
Julien Lecomte.

top

Don’t Scale Images in HTML

tag: images

Don’t use a bigger image than you need just because you can set the
width and height in HTML. If you need
<img width="100" height="100" src="mycat.jpg" alt="My Cat" />
then your image (mycat.jpg) should be 100x100px rather than a scaled
down 500x500px image.

top

Choose <link> over @import

tag: css

One of the previous best practices states that CSS should be at the top
in order to allow for progressive rendering.

In IE @import behaves the same as using <link> at the bottom of the
page, so it’s best not to use it.

top

Make favicon.ico Small and Cacheable

tag: images

The favicon.ico is an image that stays in the root of your server. It’s
a necessary evil because even if you don’t care about it the browser
will still request it, so it’s better not to respond with a
404 Not Found. Also since it’s on the same server, cookies are sent
every time it’s requested. This image also interferes with the download
sequence, for example in IE when you request extra components in the
onload, the favicon will be downloaded before these extra components.

So to mitigate the drawbacks of having a favicon.ico make sure:

  • It’s small, preferably under 1K.
  • Set Expires header with what you feel comfortable (since you cannot
    rename it if you decide to change it). You can probably safely set
    the Expires header a few months in the future. You can check the
    last modified date of your current favicon.ico to make an informed
    decision.

Imagemagick can help you create small favicons

top

Avoid Filters

tag: css

The IE-proprietary AlphaImageLoader filter aims to fix a problem with
semi-transparent true color PNGs in IE versions < 7. The problem with
this filter is that it blocks rendering and freezes the browser while
the image is being downloaded. It also increases memory consumption and
is applied per element, not per image, so the problem is multiplied.

The best approach is to avoid AlphaImageLoader completely and use
gracefully degrading PNG8 instead, which are fine in IE. If you
absolutely need AlphaImageLoader, use the underscore hack _filter as
to not penalize your IE7+ users.

top

Keep Components under 25K

tag: mobile

This restriction is related to the fact that iPhone won’t cache
components bigger than 25K. Note that this is the uncompressed size.
This is where minification is important because gzip alone may not be
sufficient.

For more information check “Performance Research, Part 5: iPhone
Cacheability – Making it Stick” by Wayne Shea and Tenni Theurer.

top

Optimize Images

tag: images

After a designer is done with creating the images for your web page,
there are still some things you can try before you FTP those images to
your web server.

  • You can check the GIFs and see if they are using a palette size
    corresponding to the number of colors in the image. Using
    imagemagick it’s easy to check
    using
    identify -verbose image.gif
    When you see an image useing 4 colors and a 256 color “slots” in the
    palette, there is room for improvement.
  • Try converting GIFs to PNGs and see if there is a saving. More often
    than not, there is. Developers often hesitate to use PNGs due to the
    limited support in browsers, but this is now a thing of the past.
    The only real problem is alpha-transparency in true color PNGs, but
    then again, GIFs are not true color and don’t support variable
    transparency either. So anything a GIF can do, a palette PNG (PNG8)
    can do too (except for animations). This simple imagemagick command
    results in totally safe-to-use PNGs:
    convert
    image.gif image.png

    “All we are saying is: Give PiNG a Chance!”
  • Run pngcrush (or any other
    PNG optimizer tool) on all your PNGs. Example:
    pngcrush image.png -rem
    alla -reduce -brute result.png
  • Run jpegtran on all your JPEGs. This tool does lossless JPEG
    operations such as rotation and can also be used to optimize and
    remove comments and other useless information (such as EXIF
    information) from your images.
    jpegtran -copy none -optimize -perfect src.jpg dest.jpg

top

Pack Components into a Multipart Document

tag: mobile

Packing components into a multipart document is like an email with
attachments, it helps you fetch several components with one HTTP request
(remember: HTTP requests are expensive). When you use this technique,
first check if the user agent supports it (iPhone does not).

Optimize CSS Sprites

tag: images

  • Arranging the images in the sprite horizontally as opposed to
    vertically usually results in a smaller file size.
  • Combining similar colors in a sprite helps you keep the color count
    low, ideally under 256 colors so to fit in a PNG8.
  • “Be mobile-friendly” and don’t leave big gaps between the images in
    a sprite. This doesn’t affect the file size as much but requires
    less memory for the user agent to decompress the image into a pixel
    map. 100×100 image is 10 thousand pixels, where 1000×1000 is 1
    million pixels

top

Avoid Empty Image src

tag: server

Image with empty string src attribute occurs more than one will
expect. It appears in two form:

  1. straight HTML

    <img src=””>

  2. JavaScript

    var img = new Image();
    img.src = “”;

 

Both forms cause the same effect: browser makes another request to your
server.

  • Internet Explorer makes a request to the directory in which the
    page is located.
  • Safari and Chrome make a request to the actual page itself.
  • Firefox 3 and earlier versions behave the same as Safari and
    Chrome, but version 3.5 addressed this issue[bug 444931] and no
    longer sends a request.
  • Opera does not do anything when an empty image src is
    encountered.

 

 

Why is this behavior bad?

  1. Cripple your servers by sending a large amount of unexpected
    traffic, especially for pages that get millions of page views per
    day.
  2. Waste server computing cycles generating a page that will never be
    viewed.
  3. Possibly corrupt user data. If you are tracking state in the
    request, either by cookies or in another way, you have the
    possibility of destroying data. Even though the image request does
    not return an image, all of the headers are read and accepted by the
    browser, including all cookies. While the rest of the response is
    thrown away, the damage may already be done.

 

 

The root cause of this behavior is the way that URI resolution is
performed in browsers. This behavior is defined in RFC 3986 – Uniform
Resource Identifiers. When an empty string is encountered as a URI, it
is considered a relative URI and is resolved according to the algorithm
defined in section 5.2. This specific example, an empty string, is
listed in section 5.4. Firefox, Safari, and Chrome are all resolving an
empty string correctly per the specification, while Internet Explorer is
resolving it incorrectly, apparently in line with an earlier version of
the specification, RFC 2396 – Uniform Resource Identifiers (this was
obsoleted by RFC 3986). So technically, the browsers are doing what they
are supposed to do to resolve relative URIs. The problem is that in this
context, the empty string is clearly unintentional.

HTML5 adds to the description of the
图片 1 tag’s src attribute to
instruct browsers not to make an additional request in section 4.8.2:

The src attribute must be present, and must contain a valid URL
referencing a non-interactive, optionally animated, image resource
that is neither paged nor scripted. If the base URI of the element is
the same as the document’s address, then the src attribute’s value
must not be the empty string.

Hopefully, browsers will not have this problem in the future.
Unfortunately, there is no such clause for <script src=””> and
<link href=””>. Maybe there is still time to make that adjustment
to ensure browsers don’t accidentally implement this behavior.

 

This rule was inspired by Yahoo!’s JavaScript guru Nicolas C. Zakas. For
more information check out his article “Empty image src can destroy your
site”.

top

 

Don’t Scale Images in HTML

tag: images

Don’t use a bigger image than you need just because you can set the
width and height in HTML. If you need
<img width="100" height="100"
src="mycat.jpg" alt="My Cat" />

then your image (mycat.jpg) should be 100x100px rather than a scaled
down 500x500px image.

top

Make favicon.ico Small and Cacheable

tag: images

The favicon.ico is an image that stays in the root of your server. It’s
a necessary evil because even if you don’t care about it the browser
will still request it, so it’s better not to respond with a
404 Not Found. Also since it’s on the same server, cookies are sent
every time it’s requested. This image also interferes with the download
sequence, for example in IE when you request extra components in the
onload, the favicon will be downloaded before these extra components.

So to mitigate the drawbacks of having a favicon.ico make sure:

  • It’s small, preferably under 1K.
  • Set Expires header with what you feel comfortable (since you cannot
    rename it if you decide to change it). You can probably safely set
    the Expires header a few months in the future. You can check the
    last modified date of your current favicon.ico to make an informed
    decision.

Imagemagick can help you create small
favicons

top

Keep Components under 25K

tag: mobile

This restriction is related to the fact that iPhone won’t cache
components bigger than 25K. Note that this is the uncompressed size.
This is where minification is important because gzip alone may not be
sufficient.

For more information check “Performance Research, Part 5: iPhone
Cacheability – Making it
Stick” by
Wayne Shea and Tenni Theurer.

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Pack Components into a Multipart Document

tag: mobile

Packing components into a multipart document is like an email with
attachments, it helps you fetch several components with one HTTP request
(remember: HTTP requests are expensive). When you use this technique,
first check if the user agent supports it (iPhone does not).

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