There is this
Mozilla bug report from 1998, which confirms that this behaviour is (was?) intended. I haven't found out if this is due to W3C specs, although the conversation in the report implies that it was the spec back then.
Edit: Seems it is also a
HTML5 spec, although that spec recommends that a missing image icon be displayed as well. As it is, a
CSS extended property has to be added (but darned if I can figure out where) to force the icon to appear.