Taking what the HTMX project has done and making it standard in browsers (without a library) would go light-years towards bringing back the hypertext focus of the web.
* HTMX build in, gives 99% of modern page interactivity with no javascript
* No javascript.
* Yes images and videos, no custom video controls.
* Simplified CSS, all layout done via flexbox? The goal is to be able to not use HTML as part of your layout, but instead have HTML model your data as closely as possible. Looking at how simplified the HTML is for modern bootstrap compared to previous generations I think this is possible.
* Simplified HTML. There are a lot of older tags that probably shouldn't get used.
Everyone has their own set of killer features though, so it's a hard problem to coordinate on.
I think something very much like that could be really powerful as a native GUI toolkit if you included an HTML5-canvas equivalent, maybe something similar to wayland's protocol, for native apps only of course.