Audio-only use still requires media element to be type HTMLVideoElement

Created by: davemevans

supportsCodec in capabilities.js requires that media element by of type HTMLVideoElement. Despite the obvious naming conventions (videoModel etc), it isn't obvious why this is since there do not appear to be calls to any specific methods or attributes which are not derived from the parent HTMLMediaElement, either in supportsCodec, or anywhere else in the application.

We have a use case and are having some success prototyping streaming audio only using DASH. Clearly we could use a video element, but that seems like a workaround to a problem which shouldn't exist.

I realise that there are a small handful of video-specific attributes and methods (getVideoPlaybackQuality springs to mind), but it seems that the instance type could be checked in those instances if required.

Is there a good reason for this, or could this check be removed?