Looks like we are collecting some useful input for ‘design-thinking’ on this project of yours of revamping the BJS GUI 
Well. In essence, rectangle is a container. My two-cents is that we would somehow need to ‘normalize’ the properties/styling of containers and controls. Which I believe is part of what you are currently working on, isn’t it?
In my opinion, HTML mesh is a ‘compromise’ side project that will never fullfil all the needs for a proper GUI. I believe we have a much stronger and relevant base with the BJS GUI. Although, it would need a complete overhaul. I think we all agree on that (including @PatrickRyan). Obviously, there’s a lack of resources to undergo everything that’s needed for such a large project. Also considering all that has been built on top of it, incl. the GUI Editor and side- Community extensions. But as an optimistic person, I believe we can still make progress extending the current (and ‘extrapolating’, building a ‘base’ for a future). A future where, ideally, the connection to css/dom UI would be a lot closer and allow for minimal ‘effort’/‘inconvenience’ to switch or extend from… (react, vue, vite…). I humbly suppose that this would be the ultimate goal, is it? 