Warning: This blog entry was written two or more years ago. Therefore, it may contain broken links, out-dated or misleading content, or information that is just plain wrong. Please read on with caution.
Host Resources on a Different Domain
As I mentioned before, browsers can only open so many connections at once. Well I left something important out. They can only open so many connections at once to a Given Domain. If you host some of your resources on a different domain to your main site then the users browser will be able to open extra connections to that domain to download the extra resources. This is why you will often see in your browser how when you open www.somesite.com it is downloading images from downloads.somesite.com (A sub domain is considered a seperate domain to the parent).
Validate Your Site Code
After all is said and done, the most important thing you can do is run your html and css though a validator. Valid code will always run faster as it reduces the work the web browser needs to do for error checking and redrawing.
One important item of validating your code is specifying the doc type and encoding. I personally use the strict encoding as often as I can. It is more work to write fully valid code, but it requires the least work I found for cross browser compatibility and makes for clean fast executing html.