[Bug 7144] I am not sure if we are ditching ALT in favour of legend. You don't make this clear here. Some of your alt examples here resemble longdesc, which I am in favour of ditching completely. I'd be interested to see your answer on whatwg list about this
[Bug 7539] Since @summary is an official part of the table element, it is no longer obsolete. Suffienct warning language exists in it's definition that this entry should be removed
[Bug 8252] HTTP caching rules are _ignored_? What's wrong with extending Cache-Control to support user-agent caching instead of coming up with an entirely new mechanism?