For accessibility compliance, you _need_ text and a DOM. Screen readers rely on HTML element semantics, ARIA attributes, and text content. The only way to make a canvas element (which is what you'd typically need to render custom UI) accessible is to have a textual fallback.
Outside accessibility there's also the issue of responsive design, huge SEO impacts, rendering performance... I'm probably missing a bunch.
Lifting a sedan with my bare hands is obviously easier than lifting a 10 wheeler, but it's still a massive problem. Rendering stuff is not the biggest issue.
Embed a browser with all that stuff into a page. Whatever APIs are missing to make the "inner browser" unable to fulfill all the requirements that the "outer browser" will eventually be filled-in by well-meaning developers who want the Javascript VM in the "outer browser" to be able to host general purpose applications.
Outside accessibility there's also the issue of responsive design, huge SEO impacts, rendering performance... I'm probably missing a bunch.
Lifting a sedan with my bare hands is obviously easier than lifting a 10 wheeler, but it's still a massive problem. Rendering stuff is not the biggest issue.