ShadowDOM which as-implemented isn’t even fully v0 complient needs to be updated to v1 spec so that it will work with Custom Elements v1.
The real problem is that in the course of Mozilla development they hooked stuff into Stylo. This is why I have assigned @wolfbeast to the issue as well. It will require some
layout/style changes that will need to be done or evolve along a non-
This will also impact SVG if we choose to include it. Could leave it for now though..