Restricting <base> URLS via CSP

Hi all,

After chatting with Adam and Mike, I'd like to propose a new CSP field for
setting a restriction on the base URL of a document. Having this provided
in a header and/or early in the page provides a bulwark against many of the
worst post-CSS HTML injection attacks, and when combined with existing CSP
1.1 directives can deny many of the worst payload smuggling attacks.

Is there appetite in the group to specify this for 1.1?

Regards