What's the definition of "all browsers" though? Are we counting IE6? Only the newest version of each major browser? Would you be expecting web users to be using a minimum browser version?
Although a cool idea, I don't see how this would mitigate any issues currently surrounding standards.
Latest release version of major browsers (Chrome, IE, FF, Opera, Safari).
Have a mobile version which tracks mobile browsers.
Once the HTML5 term was dropped (because it's a living spec), it's impossible to refer to any meaningful feature set. Of course it's all HTML, but there's so much variance in what's supported and what is not. If I can then say 'HTML 2011', that means HTML which was supported by RTM versions in 2011.
A variation on this would be to say that rather than referring to the latest RTM version of browsers, 'HTML 2011' would refer to the subset of HTML which can target 80% (or something) of worldwide users.
Although a cool idea, I don't see how this would mitigate any issues currently surrounding standards.