Re: CustomElement: Type extension on custom tag

Hi Joshua,

I implemented Custom Elements in Chrome.

In the definition construction algorithm
<http://www.w3.org/TR/custom-elements/#dfn-definition-construction-algorithm>,
step 8.2, it says:

"If BASE does not exist or is an interface for a custom element, set ERROR
to InvalidName and stop."

In this case, BASE is the x-button and it is an interface for a Custom
Element. I think this is the case you're hitting. I think Chrome roughly
matches the spec in this case.

It's another question whether the spec could be changed to allow this. As
an implementer, I would need to know if/what order each of the callbacks
for x-button and x-submit-button were called in when something happened to
an x-submit-button. There are also interesting cases like what happens if
the x-submit-button is registered before the x-button, and whether other
things are possible like type extensions that extend other type extensions.

In short, step 8.2 avoids a lot of potential complexity by limiting what
you can do with Custom Elements in this specific way.

Dominic

On Fri, Oct 17, 2014 at 7:36 AM, Joshua Peek <josh@joshpeek.com> wrote:

> Is it legal to register a type extension on top of an existing custom tag?
>
>   <x-button is="x-submit-button"></x-button>
>
>   var ButtonPrototype = Object.create(HTMLElement.prototype)
>   document.registerElement("x-button", {prototype: ButtonPrototype})
>
>   var SubmitButtonPrototype = Object.create(ButtonPrototype)
>   document.registerElement("x-submit-button", {extends: "x-button",
> prototype: SubmitButtonPrototype})
>
>
> Chrome's native registerElement doesn't seem to like this. It throws
> the following error.
>
>   DOMException: Failed to execute 'registerElement' on 'Document':
> Registration failed for type 'x-submit-button'. The tag name specified
> in 'extends' is a custom element name. Use inheritance instead.
>
>
> The current Custom Element spec doesn't say any specific about this
> situation. Is Chrome wrong? Or is this something the spec should
> explicitly disallow?
>
>