What does "implement the HTMLElement interface" means, and how such element can be not in HTML namespace?

[kojiishi]

Step 6.3 of create an element says:

If result does not implement the HTMLElement interface, throw a TypeError.

Is there a definition for this? I'm tentatively checking if the result is a wrapper of a native object of HTMLElement or its subclass. Does this match to what the spec means?

If so, how could I create an element that matches to step 6.8?

If result’s namespace is not the HTML namespace, then throw a NotSupportedError.

I created a test:

class extends HTMLElement {
  constructor() { super(); return document.createElementNS('http://www.w3.org/2000/svg', 'g'); }
}

but this fails in step 6.3.

@domenic @dominiccooney could you advise?

[annevk]

Is there a definition for this?

See whatwg/webidl#97 for adding one.

[domenic]

Is there a definition for this? I'm tentatively checking if the result is a wrapper of a native object of HTMLElement or its subclass. Does this match to what the spec means?

Yes, this sounds correct. As the spec notes, there's an open issue on trying to define this further.

If so, how could I create an element that matches to step 6.8?

I believe it is not possible to match step 6.8 given the current definition of "element interface".

However, it could eventually be true if, for example, SVG follows up on their intention to make <svg:script> be a HTMLScriptElement. In other words, there is nothing inherently tying together implementing HTMLElement and having the HTML namespace; it happens to be true in the current spec landscape, but is not guaranteed.

[kojiishi]

Thanks for the pointer Anne and clarification Domenic.

I believe it is not possible to match step 6.8 given the current definition of "element interface".

Great, I'll then keep the code but skip the test case for now.

[domenic]

Great! I'll leave this open to track adding a non-normative note. Something like "as of the time of this writing (June 2016), this is not possible, because X. However, it could become so in the future."