Fix #1264: support namespaced attribute selectors.
layout: Make "height" relative to the initial containing block for the root.

Fixes Wikipedia.
Merge pull request #1370 from pcwalton/height-containing-block

layout: Make "height" relative to the initial containing block for the root.
Merge pull request #1368 from SimonSapin/master

Fix #1264: support namespaced attribute selectors.