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.