What? An html compliant browser would apply the custom elements spec and render the custom element. If the user isn't using JavaScript, then you fall back, as

What? An html compliant browser would apply the custom elements spec and render the custom element. If the user isn't using JavaScript, then you fall back, as defined in the spec. Note that custom elements are themselves a standard, there is no "extending" here. There is just applying the web elements spec in a way some people dislike.