When someone adds or removes a style rule, or mutates the declaration block inside it, we just restyle the whole page. It'd be really nice, in the spirit of bug 1406622, to make the invalidation framework receive a set of top-level invalidations with the selectors mutated so far in the page, and use that machinery to invalidate the style in the page before processing restyles during the stylesheet flush. This mechanism would be pretty useful for stuff like SelectorText and so on, and would make mutating the `style` of targeted rules on the page much faster. It could potentially also supersede the analysis in bug 1357583, but it's not clear it'd be a win for those cases I believe...