Looking at even a good old-fashioned blog, there are really three nested things on your screen: the blog post document, the blog post-viewing application, and the browser.
Why a blog post-viewing application? In theory you could skip it and just have a plain reader view, but in practice you do want to have navigation and comments, and that's what the application part is for.
Many modern systems surface that split in the form of an API: the API is what gives you access to the underlying document.
The challenge would be making that solid accessible to the user. For example, could a browser natively have two address bars? One for the document, and one for the application that is used to access the document?
Mozilla can implement it like they they did with asm.js, after all it is a subset, not a superset.
It will then work in all browsers but (I guess) can be made to work much faster if the browser knows it can ignore the piles of old hacks that full html has to consider.
Edit: and like asm.js it might lead to something even better going forward.