It’s not a direct method, but really, you should try a) study the source code of the big ‘frameworks’ such as Bootstrap (especially if you dig into the comments

It’s not a direct method, but really, you should try a) study the source code of the big ‘frameworks’ such as Bootstrap (especially if you dig into the comments & issues / patches that fixed worse practices with better practices) & b) hit F12 to start inspecting sites that are doing things interesting that you’d like to know about (but keep in mind they might not be experts & how you might improve it).