Best Practices for Speeding Up Your Web Site (使用最优方法来加速运行你的网站)
今天看到这篇文，有空再翻译吧。先记录下来。
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
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.
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 futureExpiresheader - For dynamic components: use an appropriate
Cache-Controlheader 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
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.
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.
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.
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.
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.
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.
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.
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 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
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.
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
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:
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
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.
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.
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.
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
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).
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.
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
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.
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
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.
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
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.
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.
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.
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
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. 100x100
image is 10 thousand pixels, where 1000x1000 is 1 million pixels
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.
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
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.
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).