Re: [whatwg/dom] `MutationObserver` flag to observe mutations in (open) shadow trees to facilitate polyfills of HTML syntax (Issue #1287)

> Over in
> 
> * [observing the composed tree synchronously webqit/realdom#3](https://github.com/webqit/realdom/issues/3)
> 
> @ox-harris implements `options.subtree = 'cross-roots'` in the `realdom` lib for synchronously observing added/removed elements across shadow root boundaries, and gets us a little closer to synchronous observation of the composed tree (f.e. detect composed children, composed parent, etc).
> 
> Not only is the usage simpler than with `MutationObserver` (because [MO events fire out of order](https://github.com/whatwg/dom/issues/1111)) and lead to simpler code, but composed tree tracking code will also be as simple when that's ready (`ComposedChildrenObserver`, `ComposedParentObserver`, `ComposedMutationObserver`, etc).

Thanks @trusktr for the mention.

Re: the above, there's been a clear misfit between what I do and what the MutationObserver APi does. For example, I sometimes just want to say:

+ observer the `style` attribute (or `draggable` attribute, etc) globally. Which exactly means "also within Shadow trees".
    + do that synchronously, so that I can closely mimic native timing for the behaviour I want to attach to the element.
        + and in some of those cases: do that *ahead of* native timing; i.e. let me intercept this attribute before the DOM sees it. E.g. to rewrite animation properties in the `style` attribute before the 'animationstart` event fires.

(A reallife usecase: implementing the upcoming OOHTML proposal.)

It turns out to be extremely difficult and mostly impossible to use the MutationObserver API in those scenarios. Writing the above referenced library - [RealDOM](https://github.com/webqit/realdom) - a realtime, and more declarative, DOM API, was the logical answer. Now, while I didn't to pursue that as a proposal, perhaps some of ideas there could help in the ideation in here.

PSS: I didn't pursue that as a proposal because I was unsure how much typical my usecase was. But I think it's more of a moving story now.

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