I've been thinking about something like this for a while - a standard for an easily optimised subset of html technologies. To conform to this standard, pages would be restricted in the ways they can manipulate the DOM, have a simpler DOM, use only a small fraction of CSS properties, and not use some JS features e.g. eval, delete.

We can use the asm.js model for opting in. Browsers that support it run the pages super fast, other browsers run them just as fast as usual.

A browser engine supporting just this standard would be considerably smaller, more embeddable, and a nicer base for current webkit based apps (e.g. spotify, steam, or atom). It might also help apps that want to use something like webviews for embedding content but need to be careful with memory / performance.