It's not security related, but: Accessibility.
Yep, browsers should have screen readers built in. It's ridiculous that you have to shell out $1000+ for a JAWS license (there are alternatives, but they need work).
NVDA and Windows's Narrator are already probably good enough but JAWS is better. VoiceOver is built into macOS and iOS and is beyond good enough.
Browsers can do a better job of exposing semantics through accessibility APIs to assistive technology [0]. Browsers could also intelligently make up for the failings of web sites; e.g. when a site uses a div with a click event instead of a button element, present it as a button through the accessibility API based on heuristics. Browser rendering engines already do a lot to visually compensate for errors in HTML, they could do the same for some semantic errors.