The relevance it that W3C HTML5 standards are supposed to already be stable everywhere, while WHATWG and the browser is a guessing game of what actually works and behaves the same way everywhere.
I've never seen anyone reference it in that way, which doesn't mean nobody does, but was the basis for my wording of "little actual relevance". (Admittedly, going to either HTML spec is not something that's needed very often for most devs, since most changes happen in other specs (CSS, web platform APIs at W3C, ...) and/or are widely documented outside, but while I've had occasional discussions involving quotes from the WHATWG spec, W3C HTML5 spec hasn't been referenced at all)
For the question "is this supported widely enough", caniuse.com + your local traffic stats is in most cases more relevant than inclusion in some spec or not.
