A customized built-in element, which is defined with an extends option. These types of custom elements have a local name equal to the value passed in their extends option, and their defined name is used as the value of the is attribute, which therefore must be a valid custom element name.
Wasn't the 'is' attribute supposed to allow for nojs progressive enhancement? That's what I was thinking of anyway.
But this spec is now under reconsideration:
#662 - 509