http://www.w3.org/TR/css3-mediaqueries/#media1

The current Media Queries CR says, "Media features... can also be used without a value. For a media feature _feature_, (feature) will evaluate to true if (feature:x) will evaluate to true for a value x other than zero or zero followed by a unit identifier (i.e., other than 0, 0px, 0em, etc.)." and provides an example:

[[
For example, the ‘color’ media feature can form expressions without a value (‘(color)’), or with a value (‘(min-color: 1)’).
]

However, it also notes:

[[
Media features that are prefixed by min/max cannot be used without a value. When a media feature prefixed with min/max is used without a value it makes the media query malformed.
]]