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.
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Content | Server | Cookie | CSS | JavaScript | Images | Mobile | All
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Minimize HTTP Requests
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.
Use a Content Delivery Network
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.
Add an Expires or a Cache-Control Header
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.
Gzip Components
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.
Put Stylesheets at the Top
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.
Put Scripts at the Bottom
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.
Avoid CSS Expressions
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.
Make JavaScript and CSS External
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.
Reduce DNS Lookups
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.
Minify JavaScript and CSS
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.
Avoid Redirects
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 http://astrology.yahoo.com/astrology
results in a 301 response containing a redirect to http://astrology.yahoo.com/astrology/
(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.
Remove Duplicate Scripts
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.
Configure ETags
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
Make Ajax Cacheable
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:
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.
Flush the Buffer Early
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 -->
Use GET for AJAX Requests
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.
Post-load Components
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.
Preload Components
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
Reduce the Number of DOM Elements
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).
Split Components Across Domains
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.
Minimize the Number of iframes
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
No 404s
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.
Reduce Cookie Size
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
Use Cookie-free Domains for Components
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.
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.
Develop Smart Event Handlers
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.
Choose <link> over @import
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.
Avoid Filters
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.
Optimize Images
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
When you see an image using 4 colors and a 256 color "slots" in the palette, there is room for improvement.
identify -verbose image.gif When you see an image using 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:
"All we are saying is: Give PiNG a Chance!"
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
Optimize CSS Sprites
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. 100x100 image is 10 thousand
pixels, where 1000x1000 is 1 million pixels
Don't Scale Images in HTML
Images
Don't use a bigger image than you
need just because you can set the width and height in HTML. If you need
then your image (mycat.jpg) should be 100x100px rather than a scaled down 500x500px image.
<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.
Make favicon.ico Small and Cacheable
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
Keep Components under 25K
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.
Pack Components into a Multipart Document
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).
Avoid Empty Image src
Servers
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 = "";
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 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".
