<x-foo> syntax fails while <button is="x-foo"> succeeds

[dave]

When I define a custom element that inherits from button with the document.register syntax, it can't be created using the syntax. Creating from code with new XFoo(); works, as does .

Here's a demo:

http://jsfiddle.net/y7R5s/12/

[dave]

We should see three buttons. What we see is:

[sjmiles]

As per spec, custom elements that are derivations of existing DOM elements (other than HTMLElement) must use the is syntax.

This is necessary because various browsers have legacy code that expects the localName (e.g.) of a <button> to always be button. This is why the is syntax was invented.

[dave]

Excellent, thanks. Perhaps the Custom Elements intro page (http://www.polymer-project.org/platform/custom-elements.html) could do with mentioning that... It doesn't really make it clear:

[sjmiles]

Eek, yes, that's a problem. Thanks for pointing that out.

[rafaelw]

Closing this since it's not an MDV bug.

[ebidel]

Updated the docs to be more clear on type extension custom elements.

https://github.com/Polymer/CustomElements/commit/bf36b5a9c85d3e1cd837faab23a586aca7ee6a7c