This specifies (with some additional detail) the proposal in
whatwg#3699 (comment) .
This changes three things:
 * the inheritance of directionality
 * the inheritance of language
 * the computation of dir=auto
to account for Shadow DOM.

This builds on the work in whatwg#9452 and whatwg#9554 to refactor this section, and
builds on work by Brian Kardell, Eric Meyer, and others in whatwg#7424 and
in whatwg#9166 and work by fantasai, rniwa, smaug, MyIdShin, Brian Kardell,
and others in whatwg#3699.

Closes whatwg#9166.

Fixes whatwg#3699.

…se whose".