As jeffa and fpi have already pointed out, browsers may interpret content differently, whether that content is HTML, JavaScript, XML, CSS or anything else.

Browsers also interpret HTTP responses differently, so you should be aware of the bugs that exist in certain Web browsers.

Aside from browser compatibility issues, it's worth getting to grips with HTTP if you're writing CGI scripts, because you can use headers such as Expires, Content-Length and Last-Modified to make your site appear faster to users. The Issuing Correct HTTP headers section of the mod_perl guide is the best reference I know of for this. The examples are specific to mod_perl, but the principles can be applied to CGI, JSP, ASP or any other server-side Web programming.