It looks like Robert Sayre is implementing Gerv's Content Restrictions proposal. This will give web sites a way to specify where in the page scripts are allowed to appear. The idea is that if a site using Content Restrictions has an XSS hole (e.g. due to incorrectly escaped text or incorrectly sanitized user HTML), it won't be exploitable in browsers that support Content Restrictions.
I'd rather see anti-XSS effort focused on improving server-side libraries. For example, if web applications were written in terms of "constructing an HTML DOM tree" or "pushing and popping HTML elements", much less code would have to be audited for "remembering to escape output" (I hate that phrase). But it's hard to argue against something that seems like it will help mitigate holes in existing web applications.