Previously we always set the height of the HTML element equal to the
viewport height but now this will only happen in quirks mode as it is
intended. Otherwise the html element height will be computed as auto.
This patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.
This patch implements "Overflow Viewport Propagation" from CSS-OVERFLOW.
It fixes an issue where many websites were not scrollable because they
had `overflow: scroll` on the body element and we didn't propagate it.
