[nicalsilva]

As pcwalton said, there is a whole lot of things on the web that are very cheap to render on a GPU if done right. For these things, a well tuned gpu renderer can easily outperform a naive compositor like what you can see in most browsers. All of the caching optimizations that one gets from a compositor can be incorporated in something like webrender (and some are, like glyphs and a bunch of other stuff). Right now webrender doesn't have all of the tweaks to avoid rendering the whole page when there is a small spinning throbber, but that will be implemented eventually (there is still a lot of work going into rendering the long tail of weird and specific things right).