This is already unprefixed, changing the title to be about removal.

https://www.chromestatus.com/metrics/feature/timeline/popularity/362

Looks the usage is still high.  But we may start deprecation.

That use counter measures any access to the WebKitMutationObserver interface object, and not invoking the constructor as the name suggests.

For this, I think that httparchive analysis will be needed to determine what typical usage in the wild looks like. I did that for some Web Audio interfaces and it turned out that removal was much lower risk than it looked like from the use counters:
https://bugs.chromium.org/p/chromium/issues/detail?id=665887#c5

In a quick httparchive search, most hits are on the form window.MutationObserver||window.WebKitMutationObserver to work with some old WebKit version that didn't have the unprefixed interface.

Given that EdgeHTML doesn't have this, attempting removal seems like the way to go.

> Standard: None,
> not for the unprefixed API either.
What do you mean? For the unprefixed API, here it is -
https://dom.spec.whatwg.org/#mutationobserver

Sorry, I was thinking about mutation events.

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