[whatwg/dom] document.open()'s mutation event suprression (#640)

`document.open()`, and various other places, such as the HTML parser, suppress mutation events. Unfortunately (and per usual) it differs across browsers. Chrome/Edge/Safari appear to set a one-off flag, whereas Firefox will only suppress them between `document.open()` and `document.close()`.

That is, if you add `frame.contentDocument.body.appendChild(document.createElement("x"));` after the `close()` call in https://github.com/w3c/web-platform-tests/pull/10680, Firefox will "fail" the test.

If we ever define them in detail, see #305, we should make sure this nuance is covered.

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