Re: [whatwg/dom] Make it possible to observe connected-ness of a node via MutationObserver (#533)

@annevk 

> So by synchronous you mean equivalent timing to custom elements, not equivalent timing to mutation events? Why is mutation observer timing insufficient?

For the same reasons node reactions are sync for CE callbacks. you want to know when the element is connected/disconnected to take immediate action, from registration, to coordinations with parent, to any other thing that all these libraries and frameworks are doing today synchronous as part of the diffing algo. Good example here is the snabbdom library that powers VUE and LWC, it is one of those popular diffing libs that provides tons of sync hooks for the operations on the dom.

> Why would you need to polyfill/emulate custom elements at this point?

Polyfilling is always handy, we don't know what the future will bring :), one example here is the scoped registry, that will take a while to take shape, but having this feature in place means that we can emulate some of the custom elements reactions without registering a real CE. For the record, we do that today in LWC, but it requires many hacks to control the connect-ness, so I might be bias. But in general, having a way to emulate these low level reactions seems like a good feature to have.

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