[Tabs] Review recommendations / consider how many new elements we need

Our the initial Parts and Concepts and the bulk of our A Survey and Anaylsis of Markup Approaches attempted to survey and report information without specifics of proposals as we saw a number of things that would require more thought. Toward the end of the survey we did provide some thought and recommendations which haven't had a lot of discussion in the calls, but they were also a little fuzzy.

Since then we've also continued to think on this and I published a blog post explaining a considerably more concrete idea that several of us have been exploring (and why we didn't make other choices), including a custom element demonstrating the thrust of the idea itself - that maybe some of these 'components' are really better as affordances grantable to certain kinds of content (more similar to how scrollbars work today). Lots of details tbd there if we go that way, but we believe this splits problems for discussion in a way we don't have to have all of the answers to discuss the key points - it could (we believe) dovetail nicely with the work presented by @tabatkins and @mirisuzanne a few weeks ago.

We have asked for developer feedback in a few channels - so far we have heard only positive feedback, but it's early... Given that this is kind of unique, isn't very common in UI kits already as a markup approach and isn't even unique to tabs we'd love to present/discuss what people's feelings are as we try to solidify something into an explainer/actual proposal we can drive forward.

Start with the linked Parts and Concepts and Markup Approaches research, then read the linked Spicy Sections blog post and inspect its custom-element demonstration. Review the existing recommendations and developer feedback, and map the open questions for discussion. Done would mean agreeing on a concrete direction and solidifying it into an explainer or actual proposal.