I could also say "specific feature X is not supported in [IE6, Safari 4, completely broken due to a trunk regression in FF 18.104.22.168...]." Back when browsers had versions.
With HTML versions, I could make statements like "In HTML4, a fieldset must be accompanied by a legend, while in XHTML1.0, this requirement has been mysteriously and silently dropped." Every developer would know exactly what I meant, and how it would affect them or not. Whether it matters more for a developer, a browser (version), or just the validator, isn't really the issue, but more that we have names for things created in certain times. Versions.
While many have been promoting the idea of "living standard", which I assume is partially to encourage developers to speak on a low, per-specific -thingie/feature/bug/joe's validator/nightly build basis, it does remove our wants and needs as developers communicating with each other to be able to talk about generalities of time. Which is why on forums everywhere, developers are still saying "HTML4", "HTML5", "XHTML", "CSS3", etc. Like this thread's original question for example.
Maybe we developers are wrong to want to continue to do this, now that neither specs nor features are being released in big batches anymore, but currently this is ending up with me trying to explain stuff like this to my boss:
"Well, despite your user-hating insistence of using the new placeholder attribute in place of a label, we still need to give direction to those using IE-on-XP, Firefox before they started releasing new versions every second, Safari on Windows and on OneOfThoseBigCats-OSX, oh and those versions of Safari and Opera who support placeholders but not on textareas, Opera Mini users, and your mom's Blackberry..."
Because we're sometimes forced to explain technical things to non-technical people, where listing gritty details doesn't really help, even if it's more accurate.
What makes more sense to the boss is:
"Placeholder is an HTML5 thing, and according to our stats we still have 32% users of HTML4 browsers, so we'd better at least sneak in a label for them."
"The bugs mentioned recently on this mailing list have been fixed and commited in Orca 3.7, which finally has support for Python 3, and requires that you have Gnome 3 installed."
(or for that matter, things like "Print is not a function in Python 2.x, but is a print() function in 3.x..." this is why we have versions, so we can refer to stuff in a way that others understand).
With a versionless living standard, it's like asking "Does Firefox support x?" (answer: some firefoxes do, did you update on the 22 of March 2012?), or (and we all hear this one) "Does this software support the latest HTML?" (uh... what is the latest HTML? Everything's, like, in the flow, man)
I suspect some of us will simply resort to using actual dates when we need to discuss things like support, since version numbers are going away. I can imagine something like this:
"Well, before commit:9a7h7d231498.1.12 in May, print was not a function, but in that commit we changed it to a function, oh and we added unicode support. Pull from master if you want print as a function in your programs."