RE: Vendor-prefixes: an idea

Why not. This simplifies greatly token interpretation, and encourage to specify more than one implementation, due to the parentheses.

However, if you want to keep the philosophy I developed behind the original proposal, I would prefer something like !draft(ms), !target(ms) or !wd(ms). 

Authors shouldn't target an user agent, but a draft; even if most (if not all) draft implementations are vendor dependent in the phase during which thoses DVMs are expected to be used.