[lots] -webkit prefixed properties and values

Per the Compat Spec <https://compat.spec.whatwg.org/>, there's a
decent-sized list of CSS at-rules, values, and properties that need to
be supported with a -webkit- prefix in order to be web-compatible:
<https://compat.spec.whatwg.org/#css-compat-section>.

Since implementors have to support these in order to realistically
support web content, they should be listed alongside the features in
the relevant specs (rather than sidelined into an easy-to-miss errata
document like they currently are).

I'm planning to do this for all the specs I control.  Would others
please do the same?  The specs in question are:

* Images
* Mediaqueries
* Animations
* Transitions
* Backgrounds & Borders
* UI
* Transforms

Alternately, I'm happy to add the relevant sections to those specs.
If you'd like to wait and see what wording I come up with in my own
specs before approving me to mess with yours, that's fine too.

(The plan right now is to add a "Legacy Compatibility" appendix
defining these things.)

~TJ