Use `parse_html` in DOMParser.
auto merge of #4415 : ChrisParis/servo/parse_html, r=jdm

For https://github.com/servo/servo/issues/4362. This is a simple fix that avoids redesigning task_state. This doesn't track nesting depth, but it wasn't clear what would use the nesting depth if we tracked it. Maybe this is enough.
layout: Implement `empty-cells` per CSS 2.1 § 17.6.1.1.
auto merge of #4400 : pcwalton/servo/empty-cells, r=SimonSapin

r? @SimonSapin
layout: Implement `caption-side` per CSS 2.1 § 17.4.1.

`caption-side` is used by 4% of pages by number of loads.