A lot of pages suggest the following as a media query for smartphones:
@media only screen and (max-device-width: 480px) { ...
This is flagged with the error message "unrecognized media only".

Aside from its widespread use, I see that the W3C candidate recommendation for media queries
http://www.w3.org/TR/css3-mediaqueries/
explicitly allows such a construction (see Example IX, prefaced by the comment: "The keyword only can also be used to hide style sheets from older user agents. User agents must process media queries starting with only as if the only keyword was not present.")

So it sounds like this message is being issued erroneously. For the record, I selected CSS3 level 3 as the profile.