I was always surprised that the browser oligopoly together with the W3C already had started building a site for web platform docs - webplatform.org - and then just mothballed that in favour of MDN. Seemed weird.
Is that supposed to be for all the browsers? I always thought it was specifically for chrome and sponsored just by google. But maybe the bias towards chrome is part of why MDN became more prevalent.
Snapshot: https://webplatform.github.io