It's not XML with vector shapes, that's exactly what SVG is. Vector shapes would live all inside HSS, where you could reuse them, style them, give them flexible sizes, apply effects, etc...
Once flexbox, grids and calc come out as official specs, you'll still have to wait until all browsers implements them, and then wait again until enough users update. You'll probably have to do an old-style layout as well anyway, because there are always users with older browsers out there, and it's not as easy as with rounded corners, where if they aren't there it doesn't matter. This would break your entire page...
How would the situation be any different for AXR?
On the other hand, there will be a versioning system, so that authors can specify what version of the technology they are targeting. So if the spec changes, compatibility modules can be created akin to how Adobe apps open documents differently if they are CS3 or CS5 documents, interpreting them according to what version they are.
Usually, layers can be built with new features, where only the very basic aspects of your design require a certain feature set, while others can be optional. In some scenarios, the user will have an outdated system and the author has REQUIRED a higher version than what is installed. Then a built-in prompt to update could be displayed, maybe with an option to view anyways, with the knowledge that it may render incorrectly.
Long story short, there are many things that can be done, and we certainly will learn a lot along the way, but I think we should definitely try.
Then, this is based on doing websites in the web standards spirit of using plain-text files, where multiple components are linked together, and having the semantic content (XML) separated from the presentation (HSS).
I mean, we like the concept of HTML+CSS, but we would like to see many of its flaws and shortcomings corrected.
Flexbox is already in that bundle of browsers (and it rocks :)
calc is in IE9+ and Firefox.
Chrome and Firefox are in autoupdating very short release cycles (or, at least, Firefox is getting there rapidly).
I think the ideas in HSS are interesting, and I understand the motivation. But, the world is rapidly changing. Not everyone is going to be able to support the latest standards in their apps, but I think we're moving past the point where most developers will need to support a 10 year old browser. Time will tell!
(obDisclaimer: I'm a Mozillian)
Sorry, but HTML isn't going anywhere. XML had its chance — full support from W3C and implementations in all-but-one browsers for a decade now. text/html parsers survived that.
I don't think you can convince all browser vendors to bet on XML again. They all opposed XHTML2. W3C tried to push XHTML2 without them and ended up making HTML even stronger.
XML with vector shapes? We've got SVG. It may not be perfect, but has tools and implementations.
Easier CSS layouts? Flex box, grids and calc() are around the corner. They'll be usable sooner than HSS, and kill the project by being good enough.