That's a tag used in XHTML/CSS to define what device the page will be rendered, too. Technically, good XHTML/CSS coding practices will define the audience and tailor pages to specific formats. It's not in widespread use because of the multiple .css files a site would need to make for each media format -- making the whole purpose of using an universal stylesheet null and void.
Some tags will never be used as often as others, and they're there for specific purposes and made available. It doesn't mean everyone needs to use it.
When XML/XSLT comes along, it would even be pointless.
