@KurtCattiSchmidt, @dandclark, and Tien Mai have provided an excellent analysis/proposal loosely titled "Declarative shadow DOM style sharing".

Framed as "a starting point for engaging the community and standards bodies in developing collaborative solutions fit for standardization", as I understand it, in my words it envisions:

• a declarative form of ES module imports/exports
• for importing resources (CSS, HTML & other)
• into a declarative shadow DOM
• via the module graph

Many thanks for potentially moving forward multiple issues like #909, #910, #939, #10176 and more.

I have made some suggestions in this slide deck with examples on github.