Re: [w3c/webcomponents] [idea] Make use of `observedAttributes` optional (#565)

I agree, changing the semantics of `observedAttribute` at this stage would be contentious.  Google wanted to special case `style` and other animated attributes for performance reasons and Mozilla didn't want to special case a single attribute.  And then there was an issue of `attributeChangedCallback` being spammy when there is a lot of attributes or some attribute you don't care keep changing its value.  So the compromise was to always specify a list of attribute to observe.

Defining the list of *standard attributes* is hard because the list is not finalized.  As more attributes are added to the standard, components which relied that those attributes to be not standard will fail to observe the attribute value changes even if the attributes only had a meaning in some builtin elements.  As such, this approach poses a serious forward compatibility concern.

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