As mentioned in bug 20029, there is an issue with XBLElement inheriting from StyledElement only because XBLDivElement (class associated with the <xbl:div> tag) needs that. XBLElement should inherits from Element and XBLDivElement should be the only one inheriting from StyledElement.
Currently this is tricky to do because we need XBLDivElement to inherit also from XBLElement.
A way of solving this is by turning StyledElement to a mix-in.
We ripped this code out and a new implementation is under way.