Re: [w3c/webcomponents] "is" attribute for autonomous custom elements (#603)

Because we have the limitation on the shadow DOM spec side of not being able to attach a shadow root on the blacklisted elements? That limitation is not present on normal custom elements. I imagine that at parsing time the browser could treat a custom tag with an `is` attribute as if it were that element that is trying to extend, maybe have it's prototype be the the one of the native tag instead of `HTMLUnknownElement`, except for the part of attaching it's own shadow root and then wait for the usual upgrade process. If having no blacklisted elements in the shadow DOM spec and being able to tell the browser not to add a shadowDOM to native elements(native randering) then there is no need for this and custom built-in elements should be enough, but it doesn't look like it's going to be the case.  
As I've mentioned I don't know a lot about the the way different elements work and why cannot be extended, can you elaborate more on the two points you mentioned?
Also I'm open to ideas and alternatives, don't you think is something that should be and can be solved? (built-in custom(blacklisted) element + shadowDOM)

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