Re: [whatwg/dom] Consider adding localNameFilter to MutationObserver (#398)

>> That's simply non-starter.
>1. Why?

It would be way too slow.

> For use cases where someone will put it, it basically means that if they can't get it on the MutationObserver config to be performed (and heavily optimized) by the engine, they'll put it in the callback anyway, because their observer clearly needs it.

That's okay. Evaluating selectors after the fact is a lot more efficient especially because you'd be executing a lot of them all at once. What's extremely slow is for each DOM mutation to synchronously execute a selector to determine whether a given element should be queued to an observer or not. That's not the kind of work you don't want to be doing for every node being added or removed.

> 2. But, assuming that I'm missing something
> Is a combination of localNameFilter and attributeFilter realistic?

That's a lot more realistic filtering assuming attribute filter is about filtering based on the presence of an attribute and not its value.

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