This flag defaults to false for new Documents, such as the one created
here for use by template elements' contents. Without setting it to
true, nothing inside a template can have a declarative shadow dom.
As noted, this appears to be a spec issue. I am not convinced that this
is the correct fix, but it is simple and does solve the issue without
any apparent regressions.
If attachment fails for whatever reason (e.g the host element is not
allowed to be a host), the HTML spec tells us to insert the template
element anyway and proceed.
Before this change, we were recomputing the insertion location at this
point, which caused it to be *inside* the template element. Inserting
the template element into itself didn't work, and so the DOM would end
up incorrect.
The fix here is to simply use the insertion point we determined earlier
in the same function, before putting a template element on the stack of
open elements. We already do this elsewhere.
Fixes at least 228 subtests on WPT. :^)
