HTMLDivElement has no properties or methods not inherited from HTMLElement. If you inspect the user-agent styling of div versus any non-standard named element, you'll see that they have the same styling path, specifically unlike with span. In other words, non-standard elements and divs are all block elements with the default HTMLElement attributes and methods.

The use of HTMLElement and HTMLUnknownElement for the non-standard elements is to support future additions to the standard elements and enable subclassing down-tree instead of across-tree, which is noted in the spec.