document.stylesheets - https://developer.mozilla.org/en-US/docs/Web/API/document.st...

The main part of the injected code is https://gist.github.com/benfoxall/5647039 - there's a bit of hacking round cors issues there too.

You can use cssom (by visionmedia) and document.querySelectorAll/sizzle.

Presumably, you could take the selectors in the CSS and pass them to a JavaScript selector engine. That way, you'd automatically have a list of elements affected by each rule.