What are the alternatives? Using background-images with off-screen text? Bad code:

1. <a href="foo" class="button">
2. <span>Alternative Text</span>
3. </a>

1. a.button, button {
2. background: url(foo.gif) 0 −26px no-repeat;
3. display: block;
4. height: 52px;
5. overflow: hidden;
6. position: relative;
7. width: 150px;
8. }
9. span {
10. left: −9999px;
11. position: absolute;
12. top: auto;
13. }

That’s a bad idea with accessibility issues. Screen readers will be able to read the text, but if somebody with low vision has high-contrast custom stylesheets, it is likely that the background-image won’t be displayed. Still the alternative text will be invisible off-screen, so the link or button becomes unusable.

A better solution

Now Google’s accessibility specialist T.V. Raman explained their idea for replacing images with sprites. Here are screenshots of Google search results with images, and with background images disabled:

Google puts the alternative text behind the background image simply by attaching it on the span element, not the anchor. So when CSS or background images are turned off, the text just reappears.

1. <a id="logo" title="Go to Google Home" href="http://www.google.com/">
2. Google
3. <span></span>
4. </a>

1. #logo {
2. display: block;
3. height: 52px;
4. overflow: hidden;
5. position: relative;
6. width: 150px;
7. }
8. #logo span {
9. background: url(foo.gif) 0 −26px no-repeat;
10. height: 100%;
11. left: 0;
12. position: absolute;
13. top: 0;
14. width: 100%;
15. }

There’s only an issue if you have a lot of alternative text, perhaps combined with text zoom, so that it doesn’t fit in the reserved space. But I can live with that.

Congratulations to Google for this ellegant solution, and it’s so simple! Why didn’t I think of it?

The traditional focus of performance optimization has been on the backend, i.e. system efficiency. But comparing a number of high profile websites, the Yahoo! team found that frontend performance is responsible for 80-98% of the perceived response time. Therefore doubling the frontend performance gains more than doubling the backend performance. In case studies Yahoo! Search became 40-50% faster, the Yahoo! Mail web application gained 70-100%. Of course there are ways to increase backend performance without throwing in more hardware, but better frontend performance reduces traffic and saves resources.

Saving resources on the client side, particularly CPU usage, also pays off in speed. Event delegation is faster than a large number of event handlers. Likewise we know that reducing the number of HTTP requests through techniques like CSS sprites, sliding doors, or file aggregation increases speed. The reason is the limit of two parallel requests per host imposed by HTTP 1.1. That results in a download queue of two requests at a time, increasing the perceived response time of a page. By configuring additional host aliases for your server you can increase the number of parallel requests — but more than 2-4 also increase DNS lookups resulting in higher CPU usage and slower response times.

I wonder when Yahoo! will present us another impressive calculation how many gigawatts have been preserved by reducing CPU usage in client PCs and in their data centers, as one participant asked in the Q&A part. Energy efficient servers are the next big thing, but are there any concrete suggestions for greener programming? Is AJAX destroying the ozone layer?

Environmental issues aside, here’s the list of rules. I’ll keep it short where I have written about it in my previous article. See the examples and testcases by Steve Souders.

1. Make fewer HTTP requests: This also affects cookies. Eliminate unnecessary cookies, keep them small, set them at granular domain levels (e.g. finance.yahoo.com instead of .yahoo.com), and set an appropriate Expires date.
2. Use a content distribution network (CDN) like Akamai where your (static) content is served from distributed data centers located nearer to your client. Even if your website is not as big as Google you can profit from faster response times by using the YUI library’s own CDN.
3. Add an Expires header not just for images, but also for JavaScript and stylesheet files.
4. Enable gzip: 90%+ of browsers support compression, and gzip is better supported and compresses more than deflate. Gzip HTML files, CSS, scripts, XML, JSON — no images or PDFs.
5. Put CSS at the top, avoid @import as it loads last, even after the images!
6. Move scripts to the bottom as they block parallel downloads even across hostnames and block rendering of any code below them.
7. Avoid CSS expressions as they execute many times and cost CPU.

8. Use external JavaScript and CSS files. Inline CSS is apparently faster for a user’s start page, but not on subsequent pages. After the page has finished loading, use the time to preload scripts to speed up secondary pages.

   1. window.onload = downloadComponents;
   2. function downloadComponents() {
   3. var elem = document.createElement("script");
   4. elem.src = "http://.../file1.js";
   5. document.body.appendChild(elem);
   6. }
9. Reduce DNS lookups for the reasons stated above. Use 1-4 hosts and the keep alive setting.
10. Minify JavaScript with JSMin — inline scripts, too.
11. Avoid redirects as they are the worst form of blocking. Set Expires headers for redirects to enable caching.
12. Remove duplicate files: this is self-explanatory, but it can happen in large teams with many scripts and stylesheets.

13. Mind the ETag: Now this was something I never paid attention to. ETags are unique identifiers to distinguish files that share a URI. They are transmitted in the HTTP header. The default server setting uses the INode, the bytesize and the modification date of a file to calculate a unique ID. Unless servers in a cluster are identical, ETags differ, therefore the files are not cached. Fortunately ETags can be configured in Apache, so it should be possible to match them across different servers.

    1. FileETag MTime Size

    Note that the ETag is also relevant for RSS feeds. For example, currently the W3C talks feed is more or less unusable: some feed readers and services apparently regard the ETag, the feed is mirrored on many servers, so the same news entry from a different server is shown as new and unread multiple times every day…

14. Make AJAX cacheable and small. Some data like a user’s address book or buddy list change infrequently and should be requested via GET, cached, and set with a Last-modified timestamp and gzipped.

These are a lot of rules, and they will be published in a O’Reilly book by Steve Souders and Tenni Theurer in September 2007. Anyway, don’t be overwhelmed by their mass, instead you can start with the easy things: “harvest the low hanging fruit.” Enable caching with the Expire date setting and reduce the number of HTTP requests. You can deal with the rest later.

Finally Nate Koechley announced a Yahoo! performance tool called YSlow as a plugin for the indespensible Firebug extension. He also recommended the commercial IBM Page Detailer, and LiveHTTPHeaders to visualize what’s happening in your browser.

Parsing JavaScript freezes the browser. Therefore put CSS in the head and JavaScript near to the </body> so that it is parsed when the page has been rendered.

The arch enemy of performance are HTTP requests. Many browsers still can’t handle more than two or four requests at a time. Keep the number of files down, your website will be faster.

There are several techniques with the aim to reduce the number of files:

1. “A single large file is fastest.” (Nate Koechley) That’s why Yahoo! apparently has such an amount of inline CSS. They found out browser caching is not as effective as they thought, in particular not on a user’s start page. So they deliver “inline” CSS. Actually writing inline CSS is a maintenance nightmare, but delivering CSS content inline doesn’t mean the files can’t have separate lives on the server: concatenate the files with a server side technique of your choice.

   Update: A couple of months later Nate explained that further: when your page is likely to be a user’s start page, caching plays a minor role, thus “inline” CSS is faster. Otherwise use external files, aggregate them, and make sure they are cached (see below).

2. Enforce caching. Another IE bug prevents image caching. Add the following to your .htaccess, httpd.conf or vhost.conf settings:

   1. <IfModule mod_expires.c>
   2. ExpiresActive On
   3. ExpiresByType image/jpg "access plus 1 day"
   4. ExpiresByType image/jpeg "access plus 1 day"
   5. ExpiresByType image/gif "access plus 1 day"
   6. ExpiresByType image/png "access plus 1 day"
   7. </IfModule>

3. Reduce the number of background images with techniques like CSS Sprites or Sliding Doors. Instead of four images of rounded corners you only need one and get the mouseover state for free! The green download button on mozilla.com is based on that technique. And Yahoo! uses CSS Sprites to combine a huge number of icons.

   

   Please note this approach is only for decorational background images that degrade gracefully. It’s not for img elements. Be careful when you use it for foreground images. And if text comes as a graphical representation, it can become inaccessible for screen reader users, zoom readers, or people with stylesheets switched off. Use real text instead.

   Also note changing the background-position causes IE6 to flicker, related to the caching bug above. To avoid it, simply add the following:

   1. <script type="text/javascript">
   2. try { document.execCommand( "BackgroundImageCache", false, true); } catch(e) {};
   3. </script>

4. Aggregate files. Ed Eliot wrote a nice script to merge JavaScript or CSS files, bonus respect for the advanced versioning and caching features.

   But remember the cases when it doesn’t make sense to merge CSS files: your IE bugfixes still belong in conditional comments. If you use the @import rule to filter antique browsers from getting advanced styles, you can’t drop it. And if you want to merge stylesheets for different media (e.g. print), make sure the code is enclosed in something like

   1. @media print {
   2. /* style sheet for print goes here */
   3. }

   In an updated version Ed added JSMin to strip comments and excess whitespace. JSMin works like a charm for JavaScript files. But it cuts a few space characters too much so that the syntax of CSS selectors changes therefore for now I have abandoned the idea to compress them too. See Jens Meiert’s comment below for a recommendation to minimize CSS.

   His original code requires the C version of JSMin with PHP safe_mode turned off. If you prefer a pure PHP version, get the PHP version of JSMin and my adapted version of the script.

I’m still in awe how fast one of my own websites became! Thanks to the guys at Yahoo! for the inspiration and for most of the research this article is based upon. Even JSMin was written by an employee of Yahoo! Speaking about Yahoo! employees: Chris, I hope there are still enough topics for your Vitamin article. I wanted to write about performance anyway, and to my surprise I read yesterday that you have similar plans. See ya in Paris.