You can be certain that, when this does happen, these considerations will be dealt with. I feel very strongly that this is going to happen. The chance to execute arbitrary code on clients will be too alluring for adtech companies to ignore.
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.
