...not exactly (Score 1)

Sending an HTTP header with the prefix of x- is perfectly legal as far as I know. I think it's the recommended way of sending data that may be safely ignored by other clients. Seems like much less of an ugly kludge than doctype switching or conditional comments.

Maybe it SHOULD be a standard. I'm tired of people hoping that someday we will reach a time where all browsers will have 100% compliance with all standards and we won't need to do some sort of negotiation to sort out the behavioral differences. This will never ever happen. And if it does, then the browser as a technology would be dead and unable to evolve or improve.

IE7 was a huge disappointment, so I'm hopeful that this could be better. It seems like it could be a less-than-terrible idea (I'm withholding judgement before I say it's a good idea). Other browsers could ignore these tags, or implement them if they wanted to. If they did, we could possibly have a way to select content within a document only when it's rendered by a particular document. (I don't have time to figure it out right now, but couldn't you do selector that finds children of a body element which is adjacent to a head element which contains the correct meta tag? Maybe not. Maybe with CSS3?