Re: [w3c/webcomponents] Allow or prohibit observedAttributes to observe 'style' attribute? (#521)

Ok, let's focus on the spec side.

The style is the only attribute that:
1. is the dynamic mirror of another object model, and
2. is the only animatable attribute.

If author wants to observe style changes, s/he should observe the source object model, not the mirrored string. Houdini has the tools to do this with specific design points, such as:
* It can opt-in to observe specific CSS properties.
* It fires per animation frame (Houdini paint) or per layout cycle (Houdini layout).
* The values are passed by values, not as serialized strings, thanks to CSS Typed OM.
* It is designed to work in Worklets.

I hope you get the picture that Houdini is trying very hard to avoid style serialization in main thread, but Custom Elements allowing to observe style property undoes all these efforts.

I don't know whether we fire mutation observer in 60fps today or not, but if we did, I think it was a design done before Houdini was born. I hope we don't repeat that.

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