How to force the browser to reload cached CSS/JS files?

Question

I have noticed that some browsers (in particular, Firefox and Opera) are very zealous in using cached copies of .css and .js files, even be

Answer 1

Interesting post. Having read all the answers here combined with the fact that I have never had any problems with "bogus" query strings (which I am unsure why everyone is so reluctant to use this) I guess the solution (which removes the need for apache rewrite rules as in the accepted answer) is to compute a short HASH of the CSS file contents (instead of the file datetime) as a bogus querystring.

This would result in the following:

Of course the datetime solutions also get the job done in the case of editing a CSS file but I think it is about the css file content and not about the file datetime, so why get these mixed up?