We are now trying to apply styles in shadow trees to a shadow host which hosts the shadow trees.

So if we have no style scoped in a treescope containing the shadow host, we will use the treescope root node.

However, if we have some style scoped and the shadow host is affected by the style scoped, how should we treat styles in shadow trees?

I think, we have two choices:

Idea 1: stylesheets in shadow trees are treated as if their scoping node is a
containing treescope, style scoped always wins.

Idea 2: stylesheets in shadow trees are treated as if their scoping node is the
same as the style scoped's scoping node. The scoping node is the first one when
walking up from the shadow host.

c.f.
https://docs.google.com/document/d/1-rTZa3gtTV-IbdYVtVVX0n_rhgSWLr6j7okJhsmMH0A/edit?usp=sharing

I would like to implement Idea 2.