Nice write-up! Just wanted to mention that the section on <style>inside a shadow root being parsed on every instance is not necessarily true. Blink made an optimization long ago which parses a style tag once, caches it, and reuses the result across instances. As long as style.texConrent is identical. https://github.com/w3c/webcomponents/issues/282#issuecomment-122186756

Essentially, constructable stylesheets were the official way to standardize this optimization in other browsers.