utils.reduceCSSSelector() is not good enough

[peterbe]

utils.reduceCSSSelector() does wrong things. Yep, you can remove pseudos classes like :hover, :focus and pseudo elements from a selector, but there are pseudos like :not(), :empty, :nth-child() etc that change the meaning of selector. Another problem that part of selector after any pseudo is cutting, e.g. "a:hover img" -> "a" so if there is no any img inside a the rule is still preserved. On the one hand, that's not a big dial, because you just left more rules that needed. On the other hand, this tool tries extract as minimal CSS as possible, so I think it should be fixed. Unfortunately, this is a complex problem so I left it as is at the moment. I let you know, when I found a better solution.

From #73 (comment)

@lahmatiy Recommended we hang tight till a better solution becomes available. One, ideally based on AST instead of some hopeful regex juggling.

We might want to consider a partial improvement that'll solve some of the easier things to tackle. For example, we can maintain a white list of pseudo selectors that are valid/important and works with cheerio. E.g. :not.
Another partial improvement is to split the selector by spaces, look for pseudo classes in each string and put it back together again. E.g. a:hover img becomes ['a:hover', 'img'] becomes ['a', 'img'] ultimately becomes a img.