StylingStarted is a kind of nebulous and not-very-useful concept. The concept
that _is_ useful is whether the presshell has been initialized or not, but the
root element may not exist at that point.
So we need to make sure we that we can trigger the initial document style in both
presshell initialized _and_ ContentInserted, which has the nice effect of handling
root element reinsertions.
We also take the opportunity to make StyleDocument assert the existence of a root
element, and align the responsibility for clearing the dirty descendant bits between
document and non-document nodes.
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