This change to requiring attribution is actually fairly recent, and was made with some reluctance on the part of us editors, despite eventually agreeing it was the best path forward. See https://blog.whatwg.org/copyright-license-change
There is a typo in the third paragraph, first sentence, FYI. "Are not" is repeated.
For example, there are derivative specifications ("forks") that are not are not prominently identified as copies of WHATWG standards