Reflection proposal: Expose internal elements to be referenced from the outside

[mrego]

I'm not a fan of the name "Cross-root ARIA Reflection" that's why the title of the issue is longer than expected. I think it can be confused with ARIA attribute reflection and be misleading in some situations.

The problem is that you want to make a relationship between something that is in a shadow tree and something from outside the shadow tree, somehow exposing/exporting/reflecting that information outside of the Shadow DOM.

Anyway this issue is to discuss what's the best approach to solve this issue. In the explainer we're describing a few options. Let's discuss them here.

Once we agree on a proposal we can bikeshed about the best name and syntax for it.