Mobile MediaViewer treats "close" as "back", which means that if you open a media viewer URL on mobile and press close, it may go to a previous webpage, or close may not work at all (Android Chrome just doesn't do anything if there's no previous URL in the stack). It works well, in other words, in the common case (article->image->close), but does not work well in the less common case (visit something in your Facebook feed, a URL in an email, etc.).