Added refs to Element's DOM spec

This is a sub-task for #1660.
Added refs to live HTMLCollections issue
Fixed FIXME message as pointed out by jdm
Rename starts_*_flow to is_*_flow
Refactor cascade() and fix cascade_with_cached_declarations()

* Expand the apply() and apply_cached() templates.
  Their two invocations each were different enough
  that this improves readability IMO.
* Create computed::Context from inherited and cascaded values
  rather than computed value, as much as possible.
* Centralize this creation rather than making it per-property,
  making 'needed_for_context' not needed anymore.
* Pass a context to cascade_with_cached_declarations() rather than
  duplicate the creation code.
auto merge of #1667 : brunoabinader/servo/dom-element-refs, r=jdm

This is a sub-task for #1660.
auto merge of #1602 : pcwalton/servo/more-inlines, r=jdm

9% improvement in style recalc on the rainbow page.

r? @jdm
Ensure id attributes on elements only affect elements present in a document.
auto merge of #1710 : jdm/servo/idmap-in-doc, r=Ms2ger

...ent.
auto merge of #1706 : SimonSapin/servo/refactor-cascade, r=pcwalton

* Expand the apply() and apply_cached() templates. Their two invocations each were different enough that this improves readability IMO.
* Create computed::Context from inherited and cascaded values rather than computed value, as much as possible.
* Centralize this creation rather than making it per-property, making 'needed_for_context' not needed anymore.
* Pass a context to cascade_with_cached_declarations() rather than duplicate the creation code.

@pcwalton r? Does this hurt the optimization introduced in 0fa0940ce9f043d19036894dc4349f9a4b2767aa?

This is mostly about nicer-looking code. The only thing that was wrong ([`apply_cached(priority)` instead of `apply_cached(needed_for_context)`](https://github.com/mozilla/servo/pull/1644#discussion_r9772133)) could be fixed with a much shorted diff.
auto merge of #1693 : lpy/servo/issue1692, r=pcwalton

see #1692
Add background-image property
Impl background-image support.
Add background test file
Add a reftest for background-image
Support base_url in stylesheet
auto merge of #1631 : recrack/servo/background-image, r=SimonSapin

for #777 #800

- support(http://www.w3.org/TR/CSS21/colors.html#background)
 - background: url(foo.png)
 - background: url(data:image/png;base64...)
 - background-image: url(foo.png)
 - background-image: url(data:image/png;base64...)

- not support(http://www.w3.org/TR/css3-background/)
Reduce code duplication in CSS parsing by passing base_url everywhere.
Add a test for the base URL of external stylesheets.
auto merge of #1711 : SimonSapin/servo/background-image, r=pcwalton