[idea] Support Namespaced Web Component Registration

[bahrus]

Related issues:

#154

#488

#842

#716

This proposal doesn't necessarily contradict anything in the last one, the scoped custom registries proposal (I don't think).

I suspect something like this must have been considered, but just in case it is feasible:

I would like to avoid namespace collisions within the same shadow DOM scope, and to allow different versions of the same element name to run together, etc. It would use a new function, customElements.defineNS, with signature:

customElements.defineNS(namespace: string, name: string, constructor, options);

So for example:

customElements.defineNS('https://unpkg.com/[email protected]/my-custom-element.js', 'my-custom-element', MyCustomElement)

would register a (or another) version of MyCustomElement, which would be activated if using:

const myCustEl = document.createElementNS('https://unpkg.com/[email protected]/my-custom-element.js', 'my-custom-element');
container.appendChild(myCustEl);

or

<ns:my-custom-element xmlns:ns="https://unpkg.com/[email protected]/my-custom-element.js"></ns:my-custom-element>

If there is no global custom element already defined with name "my-custom-element", it would also register MyCustomElement in the global namespace where you could just use

<my-custom-element></my-custom-element>

as before. I.e. the first registration gets to claim the non name-spaced tag.

[rniwa]

This is a duplicate of the issue #634.