[w3c/webcomponents] Lack of implementations of customized built-in elements is hurting Web Components (#648)

The spec says we should be able to extend or at least use built-in elements as the author interface to our custom elements, but currently no browser supports this facility and indeed the polyfill libraries which might implement this part of the spec are shying away from it, even though [the community has provided the code for it.](https://github.com/webcomponents/custom-elements/pull/88)  We want to know what will happen to our already implemented web components which rely on previous versions of libraries, and when.  Not being able to count on an outcome is hurting web components overall, for example because we are not able to port our customized built-in components to the newer versions of these libraries and are not able to present them with pride.

If the mechanism for customized built-in elements is to be deprecated, please give us an alternative that achieves the same goals, and/or remove it from the spec so that we can plan / move on.

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