Many people today use Google Closure Compiler to actually "compile" their JavaScript code. The advanced mode enables it to rewrite JS to better, faster and smaller JS-code that functions the same way as the original input.
Are there any good compilers out there for HTML/CSS? It could for example compress the CSS classes like a JS-compresser does. It could remove any divs (display: none;) that no JavaScript actually displays (display: block;). It could remove whitespaces. It could combine small images into sprites. It could combine files that are included on all pages into one file.
The list just goes on and on. Do you know about any good tools for this job? (Even if it only can compress CSS classes etc.)
Concatenate your css files and you made most of the work : the cost is mostly related to the number of requests.
But for removing useless rules, there is no point in using a tool : needing one would mean you don't really understand which ones are needed and why. In other words your code is a mess you must fix because at this point it may only grow explosively.
That's the same for sprites and other optimizations : only by knowing the use flow of the application, if people come back and have some images cached, or only the ones of the main page, etc. can you know what strategy to use, even if it's almost always useful to merge all your small images of one page in a big one.