September 2016

Quote from WHATWG.

https://html.spec.whatwg.org/multipage/introduction.html#history-2

The WHATWG was based on several core principles, in particular that technologies need to be backwards compatible, that specifications and implementations need to match even if this means changing the specification rather than the implementations, and that specifications need to be detailed enough that implementations can achieve complete interoperability without reverse-engineering each other.