Depends if you are generating new CSS definitions, or converting old pages to CSS.

If old pages, I'm going to go out on a limb here, but why re-invent the wheel when you can get Tidy for HTML for several platforms, which will not only check your HTML for errors (and correct some of them), but with the -c option will replace some tags with CSS.

If new CSS definitions, then get a list of features, and multiple screens, possibly first asking for the name, then what tags it applies to, then go from there...

Just a thought....