Corresponds to d3effb701c
What a "fixed position container" is isn't clear to me, and we don't
seem to use that elsewhere, so I've left the steps using that as FIXMEs
for now.
There's no test coverage for this in WPT yet and I'm not confident
enough in the specific behaviour to write one myself. So, waiting on
https://github.com/web-platform-tests/wpt/issues/53214
HTMLElement is the only interface that includes ElementContentEditable
in the HTML specification. This makes sense, as Element is also a base
class for elements in other specifications such as SVG,
which definitely shouldn't be editable.
Also adds a test for the attribute based on what Andreas did in the
video that added it.