As specified in instantiation of custom elements that "custom tag" will take preference over "type extension", it is required to specify the same in case of createElement(...) .

 Element createElement(DOMString localName, DOMString typeExtension);

Here if localName is also a custom tag, then what will be the prototype of "Element"? Will it be of typeExtension or of localName.

Consider the following case:

var xA = doc.registerElement("x-a");
var xB = doc.registerElement("x-b");

var customElement = doc.createElement("x-a", "x-b");

What should be the prototype of customElement?

AFAIK, this will be the same case as specifying:

<x-a is="x-b"></x-a>

In this case prototype will be of "x-a".

If the above theory is correct, then web-platform-tests/custom-elements/instantiating-custom-elements/extensions-to-document-interface/create-element-interface-type-is-a-type-extension.html test case is wrong.

If this issue has been raised earlier, then please provide the link and close this bug.