Possibly, yes. Though I would not recommend implementing it as is... XUL was never versioned as far back as when I used to work with it, which is about 5 years ago. It's messy. Not easy to use. Although still easier than the new flexbox standard that evolved from it.
Anyway I'd hope for something better than XUL/XBL. The potential was always there. Mozilla the platform! —
I massage HTML and CSS professionally since 1996, and I must say I'd prefer something like "XUL2" over my beloved web standards any day. I wouldn't mind if "XUL2" was in actuality created with web components. Though seriously, CSS and DOM (HTML/XML) were created to make you suffer. That's their true purpose.
There is no easy way out of this. And rewriting the web won't work either.
https://twitter.com/elonmusk/status/748625979271045121