:heading() now matches based on a computed heading level, which is based
on the level of the tag (h1, h2, etc) and then modified by these two new
attributes.
I'm caching this heading level on HTMLHeadingElement, based on the dom
tree version. That's more invalidation than is actually needed, but it
saves us calculating it over and over when the document hasn't changed.
The failing test cases are:
- Implicit headingreset for modal dialogs which is apparently unspecced
and controversial.
- Not walking the flat tree properly. A flat tree ancestor of a
slot-assigned element is its slot, which is something we don't do
anywhere that I could find. I've made a note to look into this later.
We also don't implement the `ReflectRange` IDL attribute yet, which
means we're not clamping the read value of `headingOffset`.
Corresponds to:
e774e8e318
