Best Practices for Speeding Up Your Web
Site
I and My team have identified a number of best practices for
making web pages fast. The list includes 34 best practices divided into 7
categories.
Filter by category:
- Content
- Server
- Cookie
- CSS
- JavaScript
- Images
- All
Minimize HTTP
Requests: Content
80% of the end-user response time is spent on the front-end.
Most of this time is tied up in downloading all the components in the page:
images, style sheets, 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 style sheet. Combining files is more challenging when the
scripts and style sheets 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
Usage - Exposed!, 40-60% of daily visitors to your
site come in with an empty cache. Making your page fast for these first time
visitors is key to a better user experience.
Use a Content
Delivery Network: Server
The user's proximity to your web server has an impact on
response times. Deploying your content across multiple, geographically
dispersed servers will make your pages load faster from the user's perspective.
But where should you start?
As a first step to implementing geographically dispersed
content, don't attempt to redesign your web application to work in a
distributed architecture. Depending on the application, changing the
architecture could include daunting tasks such as synchronizing session state
and replicating database transactions across server locations. Attempts to
reduce the distance between users and your content could be delayed by, or
never pass, this application architecture step.
Remember that 80-90% of the end-user response time is spent
downloading all the components in the page: images, stylesheets, scripts,
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: Server
There are two things in this rule:
For static
components: implement
"Never expire" policy by setting far future Expires header
For dynamic
components: use an appropriate
Cache-Control header to help the browser with conditional requests
Web page designs are getting richer and richer, which means
more scripts, stylesheets, images, and Flash in the page. A first-time visitor
to your page may have to make several HTTP requests, but by using the Expires
header you make those components cacheable. This avoids unnecessary HTTP
requests on subsequent page views. Expires headers are most often used with
images, but they should be used on all components including scripts,
stylesheets, and Flash components.
Browsers (and proxies) use a cache to reduce the number and
size of HTTP requests, making web pages load faster. A web server uses the
Expires header in the HTTP response to tell the client how long a component can
be cached. This is a far future Expires header, telling the browser that this
response won't be stale until April 15, 2010.
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.
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.
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: gzipGzip 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 style sheets,
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 Style sheets
at the Top: css
While researching performance at Yahoo!, we discovered that
moving style sheets to the document HEAD makes pages appear to be loading
faster. This is because putting style sheets 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 style sheets 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
is to follow the HTML specification and load your stylesheets in the document
HEAD.
Put Scripts at the
Bottom: javascript
The problem caused by scripts is that they block parallel
downloads. The HTTP/1.1 specification suggests that browsers download no more
than two components in parallel per hostname. If you serve your images from
multiple hostnames, you can get more than two downloads to occur in parallel.
While a script is downloading, however, the browser won't start any other
downloads, even on different hostnames.
In some situations it's not easy to move scripts to the
bottom. If, for example, the script uses document. write
to insert part of the page's content, it can't be moved lower in the page.
There might also be scoping issues. In many cases, there are ways to workaround
these situations.
An alternative suggestion that often comes up is to use
deferred scripts. The DEFER attribute indicates that the script does not
contain document.write, and is a clue to browsers
that they can continue rendering. Unfortunately, Firefox
doesn't support the DEFER attribute. In Internet Explorer, the script may be
deferred, but not as much as desired. If a script can be deferred, it can also
be moved to the bottom of the page. That will make your web pages load faster.
Avoid CSS
Expressions: css
CSS expressions are a powerful (and dangerous) way to set
CSS properties dynamically. They'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.
"#F08A00" );
As shown here, the expression method accepts a JavaScript
expression. The CSS property is set to the result of evaluating the JavaScript
expression. The expression method is ignored by other browsers, so it is useful
for setting properties in Internet Explorer needed to create a consistent
experience across browsers.
The problem with expressions is that they are evaluated more
frequently than most people expect. Not only are they evaluated when the page is
rendered and resized, but also when the page is scrolled and even when the user
moves the mouse over the page. Adding a counter to the CSS expression allows us
to keep track of when and how often a CSS expression is evaluated. Moving the
mouse around the page can easily generate more than 10,000 evaluations.
One way to reduce the number of times your CSS expression is
evaluated is to use one-time expressions, where the first time the expression
is evaluated it sets the style property to an explicit value, which replaces
the CSS expression. If the style property must be set dynamically throughout
the life of the page, using event handlers instead of CSS expressions is an
alternative approach. If you must use CSS expressions, remember that they may
be evaluated thousands of times and could affect the performance of your page.
Make JavaScript
and CSS External: JavaScript, css
Many of these performance rules deal with how external
components are managed. However, before these considerations arise you should
ask a more basic question: Should JavaScript and CSS be
contained in external files, or inlined in the page
itself?
Using external files in the real world generally produces
faster pages because the JavaScript and CSS files are cached by the browser.
JavaScript and CSS that are inlined in HTML documents
get downloaded every time the HTML document is requested. This reduces the
number of HTTP requests that are needed, but increases the size of the HTML
document. On the other hand, if the JavaScript and CSS are in external files
cached by the browser, the size of the HTML document is reduced without
increasing the number of HTTP requests.
The key factor, then, is the frequency with which external
JavaScript and CSS components are cached relative to the number of HTML
documents requested. This factor, although difficult to quantify, can be gauged
using various metrics. If users on your site have multiple page views per
session and many of your pages re-use the same scripts and stylesheets, there
is a greater potential benefit from cached external files.
Many web sites fall in the middle of these metrics. For
these sites, the best solution generally is to deploy the JavaScript and CSS as
external files. The only exception where inlining is
preferable is with home pages, such as Yahoo!'s front
page and My Yahoo!. Home pages that have few (perhaps
only one) page view per session may find that inlining
JavaScript and CSS results in faster end-user response times.
For front pages that are typically the first of many page
views, there are techniques that leverage the reduction of HTTP requests that
through using external files. One such technique is to inline JavaScript and
CSS in the front page, but dynamically download the
external files after the page has finished loading. Subsequent pages would
reference the external files that should already be in the browser's cache.
Reduce DNS Lookups:
content
The Domain Name System (DNS) maps hostnames to IP addresses,
just as phonebooks map people's names to their phone numbers. When you type
www.yahoo.com into your browser, a DNS resolver
contacted by the browser returns that server's IP address. DNS has a cost. It
typically takes 20-120 milliseconds for DNS to lookup the IP address for a
given hostname. The browser can't download anything from this hostname until
the DNS lookup is completed.
DNS lookups are cached for better performance. This caching
can occur on a special caching server, maintained by the user's ISP or local
area network, but there is also caching that occurs on the individual user's
computer. The DNS information remains in the operating system's DNS cache (the
"DNS Client service" on Microsoft Windows). Most browsers have their
own caches, separate from the operating system's cache. As long as the browser
keeps a DNS record in its own cache, it doesn't bother the operating system
with a request for the record.
Internet Explorer caches DNS lookups for 30 minutes by
default, as specified by the DnsCacheTimeout registry
setting. Firefox caches DNS lookups for 1 minute,
controlled by the network.dnsCacheExpiration
configuration setting. (Fasterfox changes this to 1
hour.)
When the client's DNS cache is empty (for both the browser
and the operating system), the number of DNS lookups is equal to the number of
unique hostnames in the web page. This includes the hostnames used in the
page's URL, images, script files, stylesheets, Flash objects, etc. Reducing the
number of unique hostnames reduces the number of DNS lookups.
Reducing the number of unique hostnames has the potential to
reduce the amount of parallel downloading that takes place in the page.
Avoiding DNS lookups cuts response times, but reducing parallel downloads may
increase response times. My guideline is to split these components across at
least two but no more than four hostnames. This results
in a good compromise between reducing DNS lookups and allowing a high degree of
parallel downloads.
Minify JavaScript
and CSS: javascript, css
Minification is the practice of
removing unnecessary characters from code to reduce its size thereby improving
load times. When code is minified all comments are removed, as well as unneeded
white space characters (space, newline, and tab). In the case of JavaScript,
this improves response time performance because the size of the downloaded file
is reduced. Two popular tools for minifying JavaScript code are
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
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,
should also be minified. Even if you gzip your scripts and styles, minifying
them will still reduce the size by 5% or more. As the use and size of
JavaScript and CSS increases, so will the savings gained by minifying your
code.
Avoid Redirects:
content
Redirects are accomplished using the 301 and 302 status
codes. Here's an example of the HTTP headers in a 301 response:
Moved Permanently
http://example.com/newuri
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
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
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
server. If a domain name change is the cause of using redirects, an alternative
is to create a CNAME (a DNS record that creates an alias pointing from one
domain name to another) in combination with Alias or mod_rewrite.
Remove Duplicate
Scripts: JavaScript
It hurts performance to include the same JavaScript file
twice in one page. This isn't as unusual as you might think. A review of the
ten top
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.
type="text/javascript" src="menu_1.0.17.js"></script>An
alternative in PHP would be to create a function called insertScript.
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
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
header.
Tue, 12 Dec 2006 03:03:59 GMT
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.
us.yimg.com
If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT
"10c24bc-4ab-457e1c1f"
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
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
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
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:
Make
content
One of the cited benefits of
feedback to the user because it requests information asynchronously from the
backend web server. However, using
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
email client the user will be kept waiting for the results of an
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
important way to improve the performance of
discussed in Add an Expires or a Cache-Control Header. Some of the other rules
also apply to
Gzip Components
Reduce DNS Lookups
Minify JavaScript
Avoid Redirects
Configure ETags
Let's look at an example. A Web 2.0 email client might use
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
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
responses are created dynamically, and might only be applicable to a single
user, they can still be cached. Doing so will make your Web 2.0 apps faster.
Flush the Buffer
Early: server
When users request a page, it can take anywhere from 200 to
500ms for the backend server to stitch together the HTML page. During this
time, the browser is idle as it waits for the data to arrive. In PHP you have
the function flush(). It allows you to send your
partially ready HTML response to the browser so that the browser can start
fetching components while your backend is busy with the rest of the HTML page.
The benefit is mainly seen on busy backends or light
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:
</head>
<body>
Yahoo! search pioneered research and real user testing to
prove the benefits of using this technique.
Use
GET for
server
The Yahoo! Mail team found that when using
two-step process: sending the headers first, then sending data. So it's best to
use GET, which only takes one TCP packet to send (unless you have a lot of
cookies). The maximum URL length in IE is 2K, so if you send more than 2K data
you might not be able to use GET.
An interesting side affect is that POST without actually
posting any data behaves like GET. Based on the HTTP specs, GET is meant for
retrieving information, so it makes sense (semantically) to use GET when you're
only requesting data, as opposed to sending data to be stored server-side.
Post-load
Components: content
You can take a closer look at your page and ask yourself:
"What's absolutely required in order to render the page initially?
JavaScript is an ideal candidate for splitting before and
after the onload event. For example if you have
JavaScript code and libraries that do drag and drop and animations, those can
wait, because dragging elements on the page comes after the initial rendering.
Other places to look for candidates for post-loading include hidden content
(content that appears after a user action) and images below the fold.
Tools to help you out in your effort: YUI Image Loader
allows you to delay images below the fold and the YUI Get utility is an easy
way to include JS and CSS on the fly. For an example in the wild take a look at
Yahoo! Home Page with Firebug's Net Panel turned on.
It's good when the performance goals are inline with other
web development best practices. In this case, the idea of progressive
enhancement tells us that JavaScript, when supported, can improve the user
experience but you have to make sure the page works even without JavaScript. So
after you've made sure the page works fine, you can enhance it with some
post-loaded scripts that give you more bells and whistles such as drag and drop
and animations.
Preload Components:
content
Preload may look like the opposite of post-load, but it
actually has a different goal. By preloading components you can take advantage
of the time the browser is idle and request components (like images, styles and
scripts) you'll need in the future. This way when the user visits the next
page, you could have most of the components already in the cache and your page
will load much faster for the user.
There are actually several types of preloading:
Unconditional preload - as soon as
Check google.com for an example of how a sprite image is requested
homepage, but it is needed on the consecutive search result page.
Conditional preload - based on a user action you make an
educated guess where the user is headed next and preload accordingly. On
search.yahoo.com you can see how some extra components are requested after you
start typing in the input box.
Anticipated preload - preload in advance
before launching a redesign. It often happens after a redesign that you
hear: "The new site is cool, but it's slower than before". Part of
the problem could be that the users were visiting your old site with a full
cache, but the new one is always an empty cache experience. You can mitigate
this side effect by preloading some components before you even launched the
redesign. Your old site can use the time the browser is idle and request images
and scripts that will be used by the new site
Reduce the Number
of DOM Elements: content
A complex page means more bytes to download and it also
means slower DOM access in JavaScript. It makes a difference if you loop
through 500 or 5000 DOM elements on the page when you want to add an event
handler for example.
A high number of DOM elements can be a symptom that there's
something that should be improved with the markup of the page without
necessarily removing content. Are you using nested tables for layout purposes?
Are you throwing in more <div>s only to fix layout issues? Maybe there's
a better and more semantically correct way to do your markup.
A great help with layouts are the YUI CSS utilities:
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
And how many DOM elements are too many? Check other similar
pages that have good markup. For example the Yahoo! Home Page is a pretty busy
page and still under 700 elements (HTML tags).
Split Components
Across Domains: content
Splitting components allows you to maximize parallel
downloads. Make sure you're using not more than 2-4 domains because of the DNS
lookup penalty. For example, you can host your HTML and dynamic content on
www.example.org and split static components between static1.example.org and
static2.example.org
For more information check "Maximizing Parallel
Downloads in the
by Tenni Theurer and Patty
Chi.
Minimize the
Number of iframes: content
Iframes allow an HTML document to
be inserted in the parent document. It's important to understand how
<iframe>
pros:
Helps with slow third-party content like badges and ads
Security sandbox
Download scripts in parallel
<iframe>
cons:
Costly even if blank
Blocks page onload
Non-semantic
No 404s:
content
HTTP requests are expensive so making an HTTP request and
getting a useless response (i.e. 404 Not Found) is
totally unnecessary and will slow down the user experience without any benefit.
Some sites have helpful 404s "Did you mean X?
wastes server resources (like database, etc). Particularly bad is when the link
to an external JavaScript is wrong and the result is a 404. First, this
download will block parallel downloads. Next the browser may try to parse the
404 response body as if it were JavaScript code, trying to find something
usable in it.
Reduce Cookie Size:
cookie
HTTP cookies are used for a variety of reasons such as
authentication and personalization. Information about cookies is exchanged in
the HTTP headers between web servers and browsers. It's important to keep the
size of cookies as low as possible to minimize the impact on the user's
response time.
For more information check "When the Cookie
Crumbles" by Tenni Theurer
and Patty Chi. The take-home of this research:
Eliminate unnecessary cookies
Keep cookie sizes as low as possible to minimize the impact
on the user response time
Be mindful of setting cookies at the appropriate domain
level so other sub-domains are not affected
Set an Expires date appropriately. An earlier Expires date
or none removes the cookie sooner, improving the user response time
Use Cookie-free
Domains for Components: cookie
When the browser makes a request for a static image and
sends cookies together with the request, the server doesn't have any use for
those cookies. So they only create network traffic for no good reason. You
should make sure static components are requested with cookie-free requests.
Create a subdomain and host all your static
components there.
If your domain is www.example.org, you can host your static
components on static.example.org. However, if you've already set cookies on the
top-level domain example.org as opposed to www.example.org, then all the
requests to static.example.org will include those cookies. In this case, you
can buy a whole new domain, host your static components there, and keep this
domain cookie-free. Yahoo! uses yimg.com, YouTube
uses ytimg.com, Amazon uses images-amazon.com and so
on.
Another benefit of hosting static components on a
cookie-free domain is that some proxies might refuse to cache the components
that are requested with cookies. On a related note, if you wonder if you should
use example.org or www.example.org for your home page, consider the cookie
impact. Omitting www leaves you no choice but to write cookies to
*.example.org, so for performance reasons it's best to use the www
Minimize DOM
Access: 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
Develop Smart
Event Handlers: JavaScript
Sometimes pages feel less responsive because of too many
event handlers attached to different elements of the DOM tree which are then
executed too often. That's why using event delegation is a good approach. If
you have 10 buttons inside a div, attach only one event handler to the div
wrapper, instead of one handler for each button. Events bubble up so you'll be
able to catch the event and figure out which button it originated from.
You also don't need to wait for the onload
event in order to start doing something with the DOM tree. Often all you need
is the element you want to access to be available in the tree. You don't have
to wait for all images to be downloaded. DOMContentLoaded
is the event you might consider using instead of
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
Choose
<link> over @import: css
One of the previous best practices states that CSS should be
at the top in order to allow for progressive rendering.
In IE @import behaves the same as using <link> at the
bottom of the page, so it's best not to use it.
Avoid Filters:
css
The IE-proprietary AlphaImageLoader
filter aims to fix a problem with semi-transparent true color
that it blocks rendering and freezes the browser while the image is being
downloaded. It also increases memory consumption and is applied per element,
not per image, so the problem is multiplied.
The best approach is to avoid AlphaImageLoader
completely and use gracefully degrading PNG8 instead, which are fine in IE. If
you absolutely need AlphaImageLoader, use the
underscore hack _filter as to not penalize your IE7+ users.
Optimize Images:
images
After a designer is done with creating the images for your
web page, there are still some things you can try before you FTP those images
to your web server.
You can check the GIFs and see if
they are using a palette size corresponding to the number of colors in the
image. Using imagemagick it's easy to check using
identify -verbose
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
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
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
Optimize CSS
Arranging the images in the sprite
horizontally as opposed to vertically usually results in a smaller file size.
Combining similar colors in a sprite helps you keep the
color count low, ideally under 256 colors so to fit in a PNG8.
"Be mobile-friendly" and don't leave big gaps
between the images in a sprite. This doesn't affect the file size as much but
requires less memory for the user agent to decompress the image into a pixel
map. 100x100 image is 10 thousand pixels, where
1000x1000 is 1 million pixels
Don't Scale Images
in HTML: images
Don't use a bigger image than you need just because you can
set the width and height in HTML. If you need
<img width="100"
height="100" src="mycat.jpg"
alt="My Cat" />
then your image (
500x500px image.
Make
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
the favicon will be downloaded before these extra
components.
So to mitigate the drawbacks of having a favicon.ico
make sure:
It's small, preferably under 1K.
Set Expires header with what you feel comfortable (since you
cannot rename it if you decide to change it). You can probably safely set the
Expires header a few months in the future. You can check the last modified date
of your current favicon.ico to make an informed
decision.
Imagemagick can help you create
small favicons
Keep Components
under 25K: mobile
This restriction is related to the fact that
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
Pack Components
into a Multipart Document: mobile
Packing components into a multipart document is like an
email with attachments, it helps you fetch several components with one HTTP
request (remember: HTTP requests are expensive). When you use this technique,
first check if the user agent supports it (iPhone
does not).
