[mtmail]

Figuring out which CSS rules are actually used. I could swear I saw a tool 10 years ago that would go through pages and list unused CSS rules. Maybe it got too complex with pre-processors and CSS having its own macro language.

[DamonHD]

There are several.

I use a combination of (from npm): cleancss, PurgeCSS, UnCSS, purifyccs.

[meiraleal]

This is a solved problem with tailwind/unocss JIT.