Introduction to XPCOM for the DOM
for instance, we have the following "chain" for the nsidomhtmlanchorelement interface: nsisupports -> nsidomnode -> nsidomelement -> nsidomhtmlelement -> nsidomhtmlanchorelement.
...if we look at nsidomhtmlanchorelement (which contains the methods and properties defined by the w3c for this element), we can see that it inherits from another interface, nsidomhtmlelement: interface nsidomhtmlanchorelement : nsidomhtmlelement this means that whatever class implements the nsidomhtmlanchorelement interface will also have to implement the nsidomhtmlelement interface.
... if we look at nsidomhtmlelement, we can see that it inherits from nsidomelement, which inherits from nsidomnode, which inherits from nsisupports.

nsIDOMHTMLMediaElement
1.0 67 introduced gecko 1.9.1 inherits from: nsidomhtmlelement last changed in gecko 1.1 the nsidomhtmlmediaelement interface implements the dom htmlmediaelement interface.

nsIDOMHTMLSourceElement
last changed in gecko 1.9.1.2 inherits from: nsidomhtmlelement the source element allows authors to specify multiple media resources for media elements.