I also want to call attention to this:
> This way browsers don't have to take into account every possible CSS property or html element when doing layout or painting.
But in order to get good performance we want to take every possible CSS property into account. Performance is about being able to make simplifying assumptions (especially on the GPU, where state changes are costly). We want to make global optimizations based on the contents of the whole page instead of having everything painted using programmatic APIs, which are black boxes to us.