I haven't said throw away HTML - I've said stop standardizing it. The link tag is still fine. In cases where people want to obliterate even that, the xml-stylesheet processing instruction also lets you specify a stylesheet.
None of this is difficult. It all works in browsers today.
No, it's not new or novel, but it's exactly what I proposed... you can already mix your own tags into HTML and style and process them.
The display property is in the CSS standard, not the HTML standard. You don't need the HTML standard to use it.
The question today is whether you want to live only inside that hollow shell, or whether you'd like to look into extending it to fit your needs. CSS, JS, and WAI-ARIA will work just as well for your own markup as they work for HTML.
You're right that this shouldn't affect back-end technologies much at all. To them it's all just markup.
Part of the headache is that they're designing during the standardizing process, making their best guesses at what might work.
Part of what I hope might come from this approach is that many people can try a variety things, and then standards can catch up to what actually worked. Browser vendors have sort of done that, but their experiments tend to have much larger consequences.
I've had the flu before, but you may be happy that I'm telling XML people similar things: put down the schemas...
Nondeterminism means never having to say you are wrong.