MDN contains both guides[1] and reference documentation[0] (which I happen to have read some minutes ago). While I have seen the occasional digression on a particular history point that explains some particular design choice, I found references to be often concise, complete, to the point, and correct (even across browsers, providing details if need be). BTW w3schools apparently contains squat about MutationObserver.

[0]: https://developer.mozilla.org/en/docs/Web/API/MutationObserv...

[1]: https://developer.mozilla.org/en-US/docs/Web/Guide