转载自CSDN
因需要登陆和发送资料才给看，所以转在这里
太多了，不知道怎样转..
http://developer.yahoo.com/performance/rules.html
http://blog.csdn.net/zlb789/archive/2008/12/21/3568187.aspx
- Use a Content Delivery Network
- Add an Expires or a Cache-Control Header
- Gzip Components
- Put Stylesheets at the Top
- Put Scripts at the 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 the Buffer Early
- Use GET for AJAX Requests
- Post-load Components
- Preload Components
- Reduce the Number of DOM Elements
- Split Components Across Domains
- Minimize the Number of iframes
- No 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
- Don't Scale Images in HTML
- Make favicon.ico Small and Cacheable
- Keep Components under 25K
- Pack Components into a Multipart Document
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.
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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.
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Add an Expires or a Cache-Control Header
tag: server
There are two things in this rule:
- For static components: implement "Never expire" policy by setting far future
Expires header
- For dynamic components: use an appropriate
Cache-Control header to help the browser with conditional requests
Web page designs are getting richer and richer, which means more scripts, stylesheets, images, and Flash in the page. A first-time visitor to your page may have to make several HTTP requests, but by using the Expires header you make those components cacheable. This avoids unnecessary HTTP requests on subsequent page views. Expires headers are most often used with images, but they should be used on all components including scripts, stylesheets, and Flash components.
Browsers (and proxies) use a cache to reduce the number and size of HTTP requests, making web pages load faster. A web server uses the Expires header in the HTTP response to tell the client how long a component can be cached. This is a far future Expires header, telling the browser that this response won't be stale until April 15, 2010.
Expires: Thu, 15 Apr 2010 20:00:00 GMT
If your server is Apache, use the ExpiresDefault directive to set an expiration date relative to the current date. This example of the ExpiresDefault directive sets the Expires date 10 years out from the time of the request.
ExpiresDefault "access plus 10 years"
Keep in mind, if you use a far future Expires header you have to change the component's filename whenever the component changes. At Yahoo! we often make this step part of the build process: a version number is embedded in the component's filename, for example, yahoo_2.0.6.js.
Using a far future Expires header affects page views only after a user has already visited your site. It has no effect on the number of HTTP requests when a user visits your site for the first time and the browser's cache is empty. Therefore the impact of this performance improvement depends on how often users hit your pages with a primed cache. (A "primed cache" already contains all of the components in the page.) We measured this at Yahoo! and found the number of page views with a primed cache is 75-85%. By using a far future Expires header, you increase the number of components that are cached by the browser and re-used on subsequent page views without sending a single byte over the user's Internet connection.
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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.
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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.
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Put Scripts at the Bottom
tag: javascript
The problem caused by scripts is that they block parallel downloads. The HTTP/1.1 specification suggests that browsers download no more than two components in parallel per hostname. If you serve your images from multiple hostnames, you can get more than two downloads to occur in parallel. While a script is downloading, however, the browser won't start any other downloads, even on different hostnames.
In some situations it's not easy to move scripts to the bottom. If, for example, the script uses document.write to insert part of the page's content, it can't be moved lower in the page. There might also be scoping issues. In many cases, there are ways to workaround these situations.
An alternative suggestion that often comes up is to use deferred scripts. The DEFER attribute indicates that the script does not contain document.write, and is a clue to browsers that they can continue rendering. Unfortunately, Firefox doesn't support the DEFER attribute. In Internet Explorer, the script may be deferred, but not as much as desired. If a script can be deferred, it can also be moved to the bottom of the page. That will make your web pages load faster.
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Avoid CSS Expressions
tag: css
CSS expressions are a powerful (and dangerous) way to set CSS properties dynamically. They're supported in Internet Explorer, starting with version 5. As an example, the background color could be set to alternate every hour using CSS expressions.
background-color: expression( (new Date()).getHours()%2 ? "#B8D4FF" : "#F08A00" );
As shown here, the expression method accepts a JavaScript expression. The CSS property is set to the result of evaluating the JavaScript expression. The expression method is ignored by other browsers, so it is useful for setting properties in Internet Explorer needed to create a consistent experience across browsers.
The problem with expressions is that they are evaluated more frequently than most people expect. Not only are they evaluated when the page is rendered and resized, but also when the page is scrolled and even when the user moves the mouse over the page. Adding a counter to the CSS expression allows us to keep track of when and how often a CSS expression is evaluated. Moving the mouse around the page can easily generate more than 10,000 evaluations.
One way to reduce the number of times your CSS expression is evaluated is to use one-time expressions, where the first time the expression is evaluated it sets the style property to an explicit value, which replaces the CSS expression. If the style property must be set dynamically throughout the life of the page, using event handlers instead of CSS expressions is an alternative approach. If you must use CSS expressions, remember that they may be evaluated thousands of times and could affect the performance of your page.
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Make JavaScript and CSS External
tag: javascript, css
Many of these performance rules deal with how external components are managed. However, before these considerations arise you should ask a more basic question: Should JavaScript and CSS be contained in external files, or inlined in the page itself?
Using external files in the real world generally produces faster pages because the JavaScript and CSS files are cached by the browser. JavaScript and CSS that are inlined in HTML documents get downloaded every time the HTML document is requested. This reduces the number of HTTP requests that are needed, but increases the size of the HTML document. On the other hand, if the JavaScript and CSS are in external files cached by the browser, the size of the HTML document is reduced without increasing the number of HTTP requests.
The key factor, then, is the frequency with which external JavaScript and CSS components are cached relative to the number of HTML documents requested. This factor, although difficult to quantify, can be gauged using various metrics. If users on your site have multiple page views per session and many of your pages re-use the same scripts and stylesheets, there is a greater potential benefit from cached external files.
Many web sites fall in the middle of these metrics. For these sites, the best solution generally is to deploy the JavaScript and CSS as external files. The only exception where inlining is preferable is with home pages, such as Yahoo!'s front page and My Yahoo!. Home pages that have few (perhaps only one) page view per session may find that inlining JavaScript and CSS results in faster end-user response times.
For front pages that are typically the first of many page views, there are techniques that leverage the reduction of HTTP requests that inlining provides, as well as the caching benefits achieved through using external files. One such technique is to inline JavaScript and CSS in the front page, but dynamically download the external files after the page has finished loading. Subsequent pages would reference the external files that should already be in the browser's cache.
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Reduce DNS Lookups
tag: content
The Domain Name System (DNS) maps hostnames to IP addresses, just as phonebooks map people's names to their phone numbers. When you type www.yahoo.com into your browser, a DNS resolver contacted by the browser returns that server's IP address. DNS has a cost. It typically takes 20-120 milliseconds for DNS to lookup the IP address for a given hostname. The browser can't download anything from this hostname until the DNS lookup is completed.
DNS lookups are cached for better performance. This caching can occur on a special caching server, maintained by the user's ISP or local area network, but there is also caching that occurs on the individual user's computer. The DNS information remains in the operating system's DNS cache (the "DNS Client service" on Microsoft Windows). Most browsers have their own caches, separate from the operating system's cache. As long as the browser keeps a DNS record in its own cache, it doesn't bother the operating system with a request for the record.
Internet Explorer caches DNS lookups for 30 minutes by default, as specified by the DnsCacheTimeout registry setting. Firefox caches DNS lookups for 1 minute, controlled by the network.dnsCacheExpiration configuration setting. (Fasterfox changes this to 1 hour.)
When the client's DNS cache is empty (for both the browser and the operating system), the number of DNS lookups is equal to the number of unique hostnames in the web page. This includes the hostnames used in the page's URL, images, script files, stylesheets, Flash objects, etc. Reducing the number of unique hostnames reduces the number of DNS lookups.
Reducing the number of unique hostnames has the potential to reduce the amount of parallel downloading that takes place in the page. Avoiding DNS lookups cuts response times, but reducing parallel downloads may increase response times. My guideline is to split these components across at least two but no more than four hostnames. This results in a good compromise between reducing DNS lookups and allowing a high degree of parallel downloads.
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Minify JavaScript and CSS
tag: javascript, css
Minification is the practice of removing unnecessary characters from code to reduce its size thereby improving load times. When code is minified all comments are removed, as well as unneeded white space characters (space, newline, and tab). In the case of JavaScript, this improves response time performance because the size of the downloaded file is reduced. Two popular tools for minifying JavaScript code are JSMin and YUI Compressor. The YUI compressor can also minify CSS.
Obfuscation is an alternative optimization that can be applied to source code. It's more complex than minification and thus more likely to generate bugs as a result of the obfuscation step itself. In a survey of ten top U.S. web sites, minification achieved a 21% size reduction versus 25% for obfuscation. Although obfuscation has a higher size reduction, minifying JavaScript is less risky.
In addition to minifying external scripts and styles, inlined <script> and <style> blocks can and should also be minified. Even if you gzip your scripts and styles, minifying them will still reduce the size by 5% or more. As the use and size of JavaScript and CSS increases, so will the savings gained by minifying your code.
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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.
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Remove Duplicate Scripts
tag: javascript
It hurts performance to include the same JavaScript file twice in one page. This isn't as unusual as you might think. A review of the ten top U.S. web sites shows that two of them contain a duplicated script. Two main factors increase the odds of a script being duplicated in a single web page: team size and number of scripts. When it does happen, duplicate scripts hurt performance by creating unnecessary HTTP requests and wasted JavaScript execution.
Unnecessary HTTP requests happen in Internet Explorer, but not in Firefox. In Internet Explorer, if an external script is included twice and is not cacheable, it generates two HTTP requests during page loading. Even if the script is cacheable, extra HTTP requests occur when the user reloads the page.
In addition to generating wasteful HTTP requests, time is wasted evaluating the script multiple times. This redundant JavaScript execution happens in both Firefox and Internet Explorer, regardless of whether the script is cacheable.
One way to avoid accidentally including the same script twice is to implement a script management module in your templating system. The typical way to include a script is to use the SCRIPT tag in your HTML page.
<script type="text/javascript" src="menu_1.0.17.js"></script>
An alternative in PHP would be to create a function called insertScript.
<?php insertScript("menu.js") ?>
In addition to preventing the same script from being inserted multiple times, this function could handle other issues with scripts, such as dependency checking and adding version numbers to script filenames to support far future Expires headers.
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tag: server
Entity tags (ETags) are a mechanism that web servers and browsers use to determine whether the component in the browser's cache matches the one on the origin server. (An "entity" is another word a "component": images, scripts, stylesheets, etc.) ETags were added to provide a mechanism for validating entities that is more flexible than the last-modified date. An ETag is a string that uniquely identifies a specific version of a component. The only format constraints are that the string be quoted. The origin server specifies the component's ETag using the ETag response header.
HTTP/1.1 200 OK
Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT
ETag: "10c24bc-4ab-457e1c1f"
Content-Length: 12195
Later, if the browser has to validate a component, it uses the If-None-Match header to pass the ETag back to the origin server. If the ETags match, a 304 status code is returned reducing the response by 12195 bytes for this example.
GET /i/yahoo.gif HTTP/1.1
Host: us.yimg.com
If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT
If-None-Match: "10c24bc-4ab-457e1c1f"
HTTP/1.1 304 Not Modified
The problem with ETags is that they typically are constructed using attributes that make them unique to a specific server hosting a site. ETags won't match when a browser gets the original component from one server and later tries to validate that component on a different server, a situation that is all too common on Web sites that use a cluster of servers to handle requests. By default, both Apache and IIS embed data in the ETag that dramatically reduces the odds of the validity test succeeding on web sites with multiple servers.
The ETag format for Apache 1.3 and 2.x is inode-size-timestamp. Although a given file may reside in the same directory across multiple servers, and have the same file size, permissions, timestamp, etc., its inode is different from one server to the next.
IIS 5.0 and 6.0 have a similar issue with ETags. The format for ETags on IIS is Filetimestamp:ChangeNumber. A ChangeNumber is a counter used to track configuration changes to IIS. It's unlikely that the ChangeNumber is the same across all IIS servers behind a web site.
The end result is ETags generated by Apache and IIS for the exact same component won't match from one server to another. If the ETags don't match, the user doesn't receive the small, fast 304 response that ETags were designed for; instead, they'll get a normal 200 response along with all the data for the component. If you host your web site on just one server, this isn't a problem. But if you have multiple servers hosting your web site, and you're using Apache or IIS with the default ETag configuration, your users are getting slower pages, your servers have a higher load, you're consuming greater bandwidth, and proxies aren't caching your content efficiently. Even if your components have a far future Expires header, a conditional GET request is still made whenever the user hits Reload or Refresh.
If you're not taking advantage of the flexible validation model that ETags provide, it's better to just remove the ETag altogether. The Last-Modified header validates based on the component's timestamp. And removing the ETag reduces the size of the HTTP headers in both the response and subsequent requests. This Microsoft Support article describes how to remove ETags. In Apache, this is done by simply adding the following line to your Apache configuration file:
FileETag none
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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.
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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.
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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.
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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.
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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
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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).
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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.
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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
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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
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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
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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.
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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
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Keep Components under 25K
tag: mobile
This restriction is related to the fact that iPhone won't cache components bigger than 25K. Note that this is the uncompressed size. This is where minification is important because gzip alone may not be sufficient.
For more information check "Performance Research, Part 5: iPhone Cacheability - Making it Stick" by Wayne Shea and Tenni Theurer.
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Pack Components into a Multipart Document
tag: mobile
Packing components into a multipart document is like an email with attachments, it helps you fetch several components with one HTTP request (remember: HTTP requests are expensive). When you use this technique, first check if the user agent supports it (iPhone does not).
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原文
上个月，Yahoo!优异性能（Yahoo!'s Exceptional Performance）开发团队成员 Stoyan Stefanov 出席了蒙特利尔的2008魁北克PHP会议演讲。他提供了他们团队最新的研究成果和提高网页性能规则20条。在早先的高性能网页开发14条军规已经让大家耳熟能详，此次新增的20条更加全面，覆盖了服务器端、cookies、页面内容、JavaScript、CSS、图片、移动手机应用这七大类别。以下内容就是根据这二十条结合个人在实际开发中的理解所做的全面解读。希望对大家开发有所助益。
阅读指导：
1. 每条规则后会指明是针对上述所说的七大类别中哪个类别的优化。
2. 文中提到的一些工具在文后附注中会提供简要说明。
3. 文中经常提到“组件”这个词，这个词不同于我们程序开发中常提到的组件概念。本文中提到的“组件”特指嵌在HTML页面中图片、JavaScript脚本、CSS等静态文件。
一、尽早清除缓冲区［服务器端］
假如用户请求一个页面，而这个页面在后端服务器需要花200至500毫秒乃至更长时间才能生成最终HTML页面，这时候用户浏览器处于较长时间的、等待页面数据返回的空闲状态，用户体验不会很好。此时可以根据页面内容长短做适当分隔，将先生成的页面局部HTML缓冲内容提前发送到客户端，不必让服务器消耗内存缓冲完整个庞大的页面内容后再行输出。这种方法有益于处理后端负荷大而前端负荷轻的页面。
在HTML页面的head标签位置后是清除缓冲的好位置，因为HTML的head标签可以包括 CSS 和 JavaScript 文件，对于浏览器而言获取页面显示与后端服务器处理并行的效果较好。在PHP中有一个函数 flush()，它可以发送请求页面的局部HTML代码给浏览器，以便浏览器能先取得页面已经生成的部分HTML，同时后端服务器继续忙于处理生成页面余下的HTML。以下以此函数做个示例：
... <!-- css, js -->
</head>
<!-- 注意此处flush()是放在了head标签位置后面 -->
<?php flush(); ?>
<body>
... <!-- content -->
其他语言也有类似语法，如ASP.NET和ASP中的 Response.Flush()。
注意：在实际Web开发中，尽量减少HTTP请求次数是优化的重要方面，这条基本原则是早先14条和新增20条中很多规则的制订基础，实际上它也是14条规则中第一条也是非常重要的一条规则，但是使用尽早清除缓冲语句会增加一个页面的HTTP请求次数，这无疑是一个矛盾，因此请注意本条规则的适用范围，不要滥用它。
二、使用GET方法的AJAX请求［服务器端］
这个容易理解一些。AJAX经常要用XMLHttpRequest，但是它的POST方法在浏览器中完成需要执行两步：首先发送信息头，然后才是发送数据；而GET方法只用一个TCP数据包传递（cookies信息例外）即可，减少了一个步骤，速度会快些。
另外根据HTTP规范，GET方法就是为获取信息而生的。因此仅在请求数据而不是发送数据给服务器端存储时，使用GET方法很有意义。
要注意的是，IE中URL允许最大允许长度是2K，用GET方法发送数据时注意2K的这个限制。
三、后加载组件［页面内容］
使用该方法的意义在于：如果某个页面内容丰富多彩的话，在浏览器加载显示它时速度就不会很快。使用后加载组件的方法可以通过延迟加载一些隐藏内容来保证浏览器优先显示初始页面。
要做到这一点必须仔细观察自己的页面并且问自己：“解释生成一个完整页面，什么部分内容是开始加载时绝对必须显示的？”清楚了这个问题，那么那些余下内容和组件就可以采用后加载方法延迟生成。这样会大大加快页面显示速度。
这个技巧通常是JavaScript通过处理页面加载时的onload事件完成。例如，使用JavaScript代码和库去执行拖放动态效果操作时，这些操作可以延迟，因为拖动页面上元素的操作只能等初始页面生成完后才能发生。页面中的隐藏内容也适合用后加载方式，因为只有页面加载完毕用户才能操作决定是否显示该内容。
Yahoo!网站的首页内容繁多，观察处于隐藏状态下的内容，这些内容通常在一些选项卡一样的标签页当中，只有点击后才会加载。
只要明白该规则的优化要点后相信大家可以通过JavaScript做出自己的具体实现。Yahoo!提供了两个用于实现后加载方法的工具：
◆ YUI Image Loader：可以延迟图片显示
◆ YUI Get utility：它可以在页面加载完成后把JavaScript和CSS资源绑定到DOM上去。
以上的工具是Yahoo!的YUI库提供。
四、预加载组件［页面内容］
从文字上看预加载组件与后加载组件似乎作用相反，但实际上二者目标是完全不同的。通过预先加载组件可以充分利用浏览器的空闲时间，并且可以请求未来页面需要的组件。在这种情况下，当用户访问下一个页面时，你已经提前让大多数组件保存在缓存中，用户加载这个页面就会非常快。
预加载类型有下列三种：
1. 无条件预加载
onload事件一触发，就要马上取回一些指定的组件。可以检查google.com首页中onload事件中请求Sprite图片的例子（注：什么是Sprite图片，请参看第十六条规则）。在这个例子可以看出这个sprite图片www.google.com/images/nav_logo3.png在google.com首页本身并不需要, 但它会在随后用户搜索生成的结果页面中需要。
2. 条件预加载
根据用户操作预测用户下一步操作的方向，并据此做预加载。例如，search.yahoo.com中，在输入框中刚键入几个字符后，就会看到页面对你键入的词做出合理推测，推出几个可能要搜索的实际关键词。此方法目前谷歌（google.cn）也在使用。
3. 提前预加载
在将重新设计的网站页面发布前用此法较好。页面重新设计后常会有这样的反馈：“新站点太酷了，就是比以前慢”。原因在于用户访问旧站点是全缓存的，但新站点还没有缓存过。这时可以在发布新设计前就预加载一些新站点组件，这可以减少没有缓存的副作用。可以利用用户访问旧站点时浏览器空闲的时间请求新站点要使用的图片、脚本等。
五、减少 DOM 元素数量［页面内容］
一个复杂的页面意味着要请求下载的字节数更多，也意味着用JavaScript访问DOM速度更慢。
如何尽量减少已有页面的 DOM 元素数量呢？一个重要的思路就是不要滥用表格table和div 。很多人习惯用一些网页编辑软件去设计页面，这样会导致大量嵌套的表格或在使用语义不合法的标记。使用div要仅当它在语义上有意义时才使用它，有些开发者使用它仅仅是因为它可以被浏览器解释生成一个新行。
Yahoo! 提供了一个避免这些问题的方法——使用YUI CSS工具。grids.css 有助于整体布局设计，fonts.css 和 reset.css 有且于清除浏览器的默认格式设置。这些工具可以在Yahoo!的YUI页面中去找。
DOM元素的数量可在Firebug的Console上键入 document.getElementsByTagName('*').length 得到。
DOM 元素不超过多少才适当呢？这可以通过检查一些有良好设计的页面来感觉比较。如Yahoo! 主页访问量相当大，它的数量在700个元素（HTML标签）以下。
六、分隔组件到多个域中［页面内容］
对终端用户响应时间影响最大的就是所请求页面所含组件数量。只要浏览器缓存为空，下载每个组件需要占用额外的HTTP请求，只有缓存满时才可能不占用。
HTTP/1.1规范中建议浏览器对每一个主机名允许并发下载两个组件。默认状态下，Internet Explorer和Firefox都符合这个规范。注意：IE8.0默认允许6个并发请求。
许多网页中所有组件都从同一主机名中下载，这时不仅响应时间受并发线程数限制，同时也受该服务器CPU和带宽限制。把页面组件分布在两个主机名中，整体响应时间就会快2倍，CPU和带宽消耗也会得以分担。
七、尽量减少 HTML 标签 iframe 的使用数［页面内容］
iframe允许在父文档内插入一个HTML文档。要想高效使用iframes，理解它的工作方式很重要。
使用iframe有如下好处：
◆ 有助于减慢显示第三方标记和广告内容。
◆ 是个安全的 Sandbox。
◆ 能并发下载脚本。
但同时也有弊端：
◆ 即使iframe 内的 HTML 文档内容为空，消耗也比较高。
◆ 会阻止页面的onload事件
◆ 非语义的
八、避免404页面［页面内容］
如果做了一个HTTP请求然后得到一个无用的响应页面，不仅完全不必要而且会降低用户体验。404页面就是在没有发现指定资源时返回的页面。
一些站点提供了有益的404提示，对用户体验有好处，但这毕竟浪费了服务器资源。当链接一个外部JavaScript文件，而它又出了404错误，这尤其糟糕。首先，因为这个下载有问题会阻止并发下载；其次，即使有错浏览器仍然会尽力解析404返回的内容，看看有无JavaScript代码，尽力查找里面可用代码。