Re: [webcomponents] [Custom]: "are" custom element names ASCII characters, or MUST they be ASCII characters? (#239)

> If we want to allow more in HTML, we should use QName from xml-names per createElementNS() since Name allows things that cannot appear in browser-implemented XML. 

I don't quite follow this reasoning. Why does stuff about browser-implemented HTML impact what we do in HTML documents?

> A code point that is not [a-z-] should be enough I think to make it "I am custom".

So `<form2>` is custom? That's kind of neat.

In any case, you seem to have the best grasp on the restrictions here. With the guiding principles of:

- We don't care how restrictive we are in XML; restrict as much as you want
- We do want to allow as much as possible in HTML
- In HTML, we don't want to require the hyphen if we're using "unusual enough" characters so that we know it's a custom element anyway

would you mind taking over the writing of the exact algorithm? Maybe even do it as a PR after #405 lands.

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