Find out what CSS styles are being used on a given page
css, http, styles
Solution
I think this website does what you want: CSS Trashman. It takes a little while to run, but it works. It reduced my personal site's CSS from 3.31 KB to 402 Bytes.
If you would rather use a command-line tool, CSS Rationator powers CSS Trashman.
Problem
I have a page that contains multiple stylesheets and a large majority of the styles in it aren't actually used in the page (deprecated etc). I would like to export a new stylesheet containing all styles on the page that are actually in use. Is there anyway to do this?