If there is no standard, what does a "conforming user-agent" conform to?
There is a standard for HTML, but it's a "Living Standard"
[wiki.whatwg.org] so it's updated regularly instead of once every 15 years or so
There's a world of difference between This browser does everything that CSS 2.1 says a browser is supposed to do and This browser does everything that was posted in such-and-such location on such-and-such date, but we make no promises about rules from the day before or the day after.
Implementers use the most up-to-date versions of standards i.e. the specs with the fewest bugs. Using old, superseded specs, would mean baking-in old bugs that have been fixed in newer versions of the specs.
For example, imagine there was some heinous security bug in CSS2.1: no implementer would deliberately release a browser with a security hole just to conform with CSS2.1.
See also: Does that mean the specification can change at any time?
It helps to think of Web "standards" as more of an ongoing, collaborative process
for implementers, developers and anoraks to develop patent-free web tech, rather than set-in-stone Standards
, like how long a standard unit of length is.
But things are falling off the far end too. One day you wake up and discover that <tt> and <u> are gone. And next week your browser says They're no longer in the standard, so I don't have to recognize them.
The HTML spec defines two things: actual technical stuff that needs to be implemented by browsers to browse the web (e.g. <font> tags, to correctly display old pages), and conformance/validation/ideology stuff for authors (e.g. use CSS instead of obsolete <font> tags). It's quite rare for browsers to actually remove stuff.
So just because a feature becomes deprecated/obsolete doesn't mean that browsers will remove it. e.g. <font> will show as an error in the validator, but is still supported in browsers.
IF everyone stopped using the <font> tag, and an army of volunteers went back and re-wrote the bazillion unmaintained pages currently using it, THEN browsers (and specs) could remove it.