Thread:Talk:Requests for comment/Scoping site CSS/Performance impact

So the idea is to generate descendant selectors? Does that have a performance impact on the client side? Browsers might have some sort of optimisation for plain class and ID selectors, and sometimes there are a very large number elements selected by site CSS in an article. I know we've previously been burnt by attempting to use child selectors, which turned out to be ridiculously inefficient in Firefox at the time.