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Not very sure if it good tool, for sites with one css file applying to all pages.

Very possible, some css rules are only used in 1 or 2 pages.

If you CAN use it for one page at a time, excellent. If not, you have to be more careful.

Here's an example of using it on just one page at a time http://www.peterbe.com/plog/mincss-in-action