CSS selector :enabled no longer matches Anchor, Area & Link elements

HTML spec has been modified [1] to disable support for :enabled CSS
selector on Anchor, Area & Link elements, after discussion on W3C
Bugzilla [2].

[1] https://html5.org/r/8818
[2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=26622
auto merge of #3491 : brunoabinader/servo/css-selector-enabled-remove-anchor-area-link-elems, r=Ms2ger

HTML spec has been modified [1] to disable support for :enabled CSS
selector on Anchor, Area & Link elements, after discussion on W3C
Bugzilla [2].

[1] https://html5.org/r/8818
[2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=26622

Next step: Move :enabled CSS selector content test to web-platform-tests.
Add support for <super> and <sup> tags. Fixes equations on wikipedia.
auto merge of #3523 : glennw/servo/wiki-equations, r=pcwalton
Pass a viewport to the rendering code

Part of servo/rust-layers#114
auto merge of #3525 : mbrubeck/servo/viewport, r=zwarich

Updates to the latest rust-layers and rust-geom to pick up
servo/rust-layers#114.  r? @zwarich
Mask root layer rectangles to their boundaries

This prevents iframes contents from overflowing their root layers.
auto merge of #3526 : mrobinson/servo/masks-to-bounds, r=zwarich

This prevents iframes contents from overflowing their root layers.
Add the defenition of InvalidNodeTypeError.
Implement some Range attributes.

  * Range.startContainer
  * Range.startOffset
  * Range.endContainer
  * Range.endOffset
  * Range.collapsed
Add NodeHelpers.length().
Add NodeHelpers.get_tree_root().
Implement Range.setStart(), Range.setEnd().
Implement Range.setStartBefore(), Range.setStartAfter(), Range.setEndBefore(), and Range.setEndAfter().
Add the test for Range's initial state.