I think the answer is this:
Browsers are allowed and encouraged to parse media descriptors – no matter what the descriptor – as a way to make them future friendly
Future versions of HTML may introduce
new values and may allow parameterized
values.
*From: http://www.w3.org/TR/html4/types.html#h-6.13
In this way, media may one day include 3d-glasses or other descriptors, including bork 😉
EDIT:
The latest CSS3 spec on media queries says this, which supports the above, to a certain degree:
Unknown media
types evaluate to false. Effectively,
they are treated identically to known
media types that do not match the
media type of the device.
*From: http://dev.w3.org/csswg/css3-mediaqueries/#error-handling
So they are treated as known and downloaded to be used, just not at that time/for that device.