> Ever really built an accessible Web-UI?

Yes. That's what I currently get paid to do.

> I mean with all that ARIA stuff, that really works for the blind or other disabled people?

Yes. If I didn't do that then we'd be sued by our customers for not being accessible.

> An accessible UI that honors the platform specific conventions and integrates into the OS environment?

Web-applications run in the browser, independent of the host OS. While web-applications do tend to adopt some OS conventions (such as having a "close popup" button in the top-left or top-right corner, web applications on-the-whole don't really ape after Windows or macOS' conventions and the past decade of post-Windows/post-macOS web-applications UX development shows that following desktop UI conventions really isn't necessary.

> Ever built an user-experience atop ideas like "form follows function" and "don't make me think", that has beautiful design at the same time?

Again, yes. (Beauty is subjective, of course - I've been told my work is "gorgeous" by my boss, so I've got at least one endorsement there)

> I actually question that there are more than a handful Web-UIs been built in that way, ever, IF actually any!

Fewer than I'd have liked, I admit. At least with today's "cleaner" look it's easier to visually design something that isn't an eyesore.

I appreciate a huge problem is that since the 2010s the learning-curve for front-end web-work has steepened considerably, and other barriers-to-entry have been introduced with no sign of them going away. Since the 2010s, when SPAs and modern CSS (flexbox, grid, responsive layout) came into being you might have noticed that fewer web dev clients today are making remarks like "my 12yo nephew could have made that!". Not to sound nostalgic, but in the late-1990s through the mid-2000s one could very easily be a very competent web-designer getting-by in Photoshop, Fireworks and Dreamweaver without ever needing to learn HTML and JavaScript (not that there was much to JavaScript back then either) - today it's increasingly essential to have at least an undergraduate CS degree to even understand how to correctly use React.js or Angular - and be comfortable with a command-line terminal. Project requirements for resolution-independent and form-factor-independent ("responsive design" - though I detest the term) web design means that the days of fixed-layout Photoshop-first web-design work and the age-old Image Slicing tool are over: tools originally designed for WYSIWYG print layout and graphic design are insufficiently expressive to allow someone to intuitively visually define how a CSS-driven layout and document-flow should work. In ye olden days it was straightforward for a relative beginner to grok how <table> could be used to make a semi-flexible layout - but explaining how CSS's grid's `auto-fit` works to the same person is impossible. And so on and so on.

Web-design has gone from the approachable place where COBOL was ("write business rules in plain-English!") to being a hallowed and exclusive techno-religious cult. I can't personally complain because I appreciate the job security I now have, but I am concerned it may lead to the web becoming less and less accessible to independent self-publishers who instead flock to much easier-to-use, but proprietary, walled-gardens like Facebook pages - or to web platforms that hide the complexity entirely, like Wix, and SquareSpace.