This revision changes the behavior of the Role function such that, for child
roles (in parent/child role relationships), the parent role must be present as
an ancestor for the child to have its assigned role. For instance, a "row" node
must have a table in its ancestry tree. To implement this, we walk parents. This
revision also fixes up and removes expected failures for around 18 web platform
tests.
Differential Revision: https://phabricator.services.mozilla.com/D202542
9f61225ecc