This is what I've been saying for a long time. HTML remains too primitive in its support for rich UI apps. The main problem I think is what you have identified

This is what I've been saying for a long time. HTML remains too primitive in its support for rich UI apps. The main problem I think is what you have identified: we need sophisticated composable widgets with practical and sensible defaults, that are also really customizable with scoped CSS, built right into the browser. This alone would be the biggest QOL improvement as far as web development goes, in several generations. I really don't know why this is not the highest priority for the W3C