This implements the DOM tree accessors that return a HTMLCollection
Fix some implementations for DOM tree accessors
Factor out CSS length to Au conversion
auto merge of #659 : sonwow/servo/dom, r=jdm

Basic implementation about DOM tree accessors in HTMLDocument.
(image, embeds, plugins, links, forms, scripts, anchors, applets)
auto merge of #663 : sanxiyn/servo/from-length, r=metajack
Simplify HTMLCollection predicates
auto merge of #666 : kmcallister/servo/htmldocument, r=jdm
Fill out various proxy binding traps as part of making setters work. Add named getter and setter and indexed getter support, as well as proxy object expandos. Fixes #660.
Implement clear
Update submodules
auto merge of #672 : jdm/servo/proxysetter, r=metajack

... named getter and setter and indexed getter support, as well as proxy object expandos. Fixes #660.
auto merge of #673 : sanxiyn/servo/clear, r=metajack
Place float children of floats correctly in the flow tree.
Fix the handling of flow_contexts in floats.

The num_floats was hard-coded to 1 and didn't take into account the
children. Also the float context was not being threaded through the children
properly.
auto merge of #676 : metajack/servo/acid1-all-boxes, r=eric93

This makes acid1 have all 6 boxes show up, and almost correctly positioned.

r? @eric93