I think the trick might be to make a browser that supports a reasonable subset of html/css/js + some killer feature. (Much easier said than done, though: but some kind of mobile app platform like WeChat might be a good way to get a foothold)

Or just fork webkit/blink and aggressively refactor.

The killer feature could be a file type named after the kinds of websites people make. Say .blog or .forum or .store or .social perhaps even .aggregator or .hn (lol) each would work like a straight jacket or like a website on a platform (.blog like blogger, .store like shopify etc) it could be distributed and insanely fast. Every other link just opens in the normal browser.

It's interesting to think about this sort of thing: interactive forms generated from OpenAPI specs? Some way to plug-in handlers for specific mime-types? etc.

This is the dream, honestly. Dynamic form creation from specs. PowerShell has something like it (for a function it will create you a ugly form that you can render), but if you could deliver a OpenAPI spec and have Bootstrap (or whatever) UI on the frontend automatically generated.

The Swagger UI[0] is more a documentation/exploration tool for APIs, but it shows enough of what you can do that I don't think it'd be all that difficult to generate something that you'd want to put on your own website.

[0] https://swagger.io/tools/swagger-ui/

This is not quite what you mean, but I've used it before to decent success:

https://github.com/rjsf-team/react-jsonschema-form

Surely IE6 already had a "reasonable subset" and devs hated it for holding back progress...

The main reason I disliked it was it wasn't standards compliant for the parts it did implement: you couldn't write code to-spec and expect it to work on both IE6 and Firefox, without testing each browser separately.

Thankfully Google has fixed that problem, ChromeOS is now the stardard that drives the Web. /s