Add documentation about the scrolling model
auto merge of #3634 : mrobinson/servo/scrolling-doc, r=jdm
layout: Introduce support for legacy presentational attributes to selector
matching, and use it for `<input size>` and `<td width>`.

This implements a general framework for legacy presentational attributes
to the DOM and style calculation, so that adding more of them later will
be straightforward.
auto merge of #3622 : pcwalton/servo/html4ever, r=jdm

This implements a general framework for legacy presentational attributes
to the DOM and style calculation, so that adding more of them later will
be straightforward.
Merge remote-tracking branch 'origin/master' into tables

Conflicts:
	components/layout/block.rs
	components/layout/fragment.rs
	components/layout/inline.rs
	components/layout/table.rs
	components/layout/table_cell.rs
	components/layout/table_row.rs
	components/layout/table_rowgroup.rs
	components/layout/table_wrapper.rs
	tests/ref/basic.list