转自：Best Practices for Speeding Up Your Web Site
Best Practices for Speeding Up Your Web Site
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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Filter by category:
- Content
- Server
- Cookie
- CSS
- JavaScript
- Images
- Mobile
- All
- Make Fewer HTTP Requests
- Use a Content Delivery Network (CDN)
- Add Expires or Cache-Control Header
- Gzip Components
- Put Stylesheets at Top
- Put Scripts at Bottom
- Avoid CSS Expressions
- Make JavaScript and CSS External
- Reduce DNS Lookups
- Minify JavaScript and CSS
- Avoid Redirects
- Remove Duplicate Scripts
- Configure ETags
- Make Ajax Cacheable
- Flush Buffer Early
- Use GET for Ajax Requests
- Postload Components
- Preload Components
- Reduce the Number of DOM Elements
- Split Components Across Domains
- Minimize Number of iframes
- Avoid 404s
- Reduce Cookie Size
- Use Cookie-Free Domains for Components
- Minimize DOM Access
- Develop Smart Event Handlers
- Choose <link> Over @import
- Avoid Filters
- Optimize Images
- Optimize CSS Sprites
- Do Not Scale Images in HTML
- Make favicon.ico Small and Cacheable
- Keep Components Under 25 KB
- Pack Components Into a Multipart Document
- Avoid Empty Image src
Minimize HTTP Requests
tag: content
80% of the end-user response time is spent on the front-end. Most of this time is tied up in downloading all the components in the page: images, stylesheets, scripts, Flash, etc. Reducing the number of components in turn reduces the number of HTTP requests required to render the page. This is the key to faster pages.
One way to reduce the number of components in the page is to simplify the page‘s design. But is there a way to build pages with richer content while also achieving fast response times? Here are some techniques for reducing the number of HTTP requests, while still supporting rich page designs.
Combined files are a way to reduce the number of HTTP requests by combining all scripts into a single script, and similarly combining all CSS into a single stylesheet. Combining files is more challenging when the scripts and stylesheets vary from page to page, but making this part of your release process improves response times.
CSS Sprites are the preferred method for reducing the number of image requests. Combine your background images into a single image and use the CSS background-image and background-position properties to display the desired image segment.
Image maps combine multiple images into a single image. The overall size is about the same, but reducing the number of HTTP requests speeds up the page. Image maps only work if the images are contiguous in the page, such as a navigation bar. Defining the coordinates of image maps can be tedious and error prone. Using image maps for navigation is not accessible too, so it‘s not recommended.
Inline images use the data: URL scheme to embed the image data in the actual page. This can increase the size of your HTML document. Combining inline images into your (cached) stylesheets is a way to reduce HTTP requests and avoid increasing the size of your pages. Inline images are not yet supported across all major browsers.
Reducing the number of HTTP requests in your page is the place to start. This is the most important guideline for improving performance for first time visitors. As described in Tenni Theurer‘s blog post Browser Cache Usage - Exposed!, 40-60% of daily visitors to your site come in with an empty cache. Making your page fast for these first time visitors is key to a better user experience.
Use a Content Delivery Network
tag: server
The user‘s proximity to your web server has an impact on response times. Deploying your content across multiple, geographically dispersed servers will make your pages load faster from the user‘s perspective. But where should you start?
As a first step to implementing geographically dispersed content, don‘t attempt to redesign your web application to work in a distributed architecture. Depending on the application, changing the architecture could include daunting tasks such as synchronizing session state and replicating database transactions across server locations. Attempts to reduce the distance between users and your content could be delayed by, or never pass, this application architecture step.
Remember that 80-90% of the end-user response time is spent downloading all the components in the page: images, stylesheets, scripts, Flash, etc. This is the Performance Golden Rule. Rather than starting with the difficult task of redesigning your application architecture, it‘s better to first disperse your static content. This not only achieves a bigger reduction in response times, but it‘s easier thanks to content delivery networks.
A content delivery network (CDN) is a collection of web servers distributed across multiple locations to deliver content more efficiently to users. The server selected for delivering content to a specific user is typically based on a measure of network proximity. For example, the server with the fewest network hops or the server with the quickest response time is chosen.
Some large Internet companies own their own CDN, but it‘s cost-effective to use a CDN service provider, such as Akamai Technologies, EdgeCast, or level3. For start-up companies and private web sites, the cost of a CDN service can be prohibitive, but as your target audience grows larger and becomes more global, a CDN is necessary to achieve fast response times. At Yahoo!, properties that moved static content off their application web servers to a CDN (both 3rd party as mentioned above as well as Yahoo’s own CDN) improved end-user response times by 20% or more. Switching to a CDN is a relatively easy code change that will dramatically improve the speed of your web site.
Add an Expires or a Cache-Control Header
tag: server
There are two aspects to this rule:
- For static components: implement "Never expire" policy by setting far future
Expiresheader - 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 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
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 usingidentify -verbose image.gifWhen 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:
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).
Avoid Empty Image src
tag: server
Image with empty string src attribute occurs more than one will expect. It appears in two form:
- straight HTML
<img src="">
- JavaScript
var img = new Image();
img.src = "";
Both forms cause the same effect: browser makes another request to your server.
- Internet Explorer makes a request to the directory in which the page is located.
- Safari and Chrome make a request to the actual page itself.
- Firefox 3 and earlier versions behave the same as Safari and Chrome, but version 3.5 addressed this issue[bug 444931] and no longer sends a request.
- Opera does not do anything when an empty image src is encountered.
Why is this behavior bad?
- Cripple your servers by sending a large amount
of unexpected traffic, especially for pages that get millions of page
views per day. - Waste server computing cycles generating a page that will never be viewed.
- 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".