Unfortunately the custom element polyfill provided by @webcomponents does not support extended built-in custom elements, has this known limitations and is slow.

The most bullet proof and battle tested polyfills I found are from @WebReflection. Please consider to at least mention them, if not just replacing by those:

• document-register-element (supports V1 despites it's name) has fantastic browser support and a public test page, please note the constructor caveat.
• built-in-element again fantastic browser support and live test page, please note the constructor caveat.