I've always wondered why Electron apps can't just share a Chrome library. That would be an improvement. Or install Electron the same way you install Java, then run html/js/css packages on top of that like an APK on Android. I guess one downside is people not updating it and distros not supplying updates in standard repos in a timely manner or never on LTS.

while not actually embedding a whole browser.

If I were to start from scratch, the browser would be much simpler. Just a handful of tags, like <block>, <inline>, <input>, <embed>, <meta>, etc. No more semantic rules, just bare building blocks that you style. That would make the DOM/engine much simpler and less memory extensive, yet you could build the exact same things you do now with Electron/Chromium via CSS and JS.

I can only speculate, but I'll bet a ton of resources (storage, ram, proc) could be saved by limiting elements and dropping semantics. You can still get semantics via attributes. Mark your main article that way, mark your menu that way, give titles and descriptions to visual things, etc. Could still be accessible, possibly even more so, for screen-readers, read-modes, and scrapers and such. Just don't make that a job of the browser.