Nitpick: If a nonstandard element name contains a hyphen (and is otherwise a syntactically valid custom element name), the element's DOM object is an instance of HTMLElement, not HTMLUnknownElement.

Quoth the standard: "The use of HTMLElement instead of HTMLUnknownElement in the case of valid custom element names is done to ensure that any potential future upgrades only cause a linear transition of the element's prototype chain, from HTMLElement to a subclass, instead of a lateral one, from HTMLUnknownElement to an unrelated subclass."