If you attempt to validate the stylesheet
http://jeffcarr.info/stylesheets/default.css, you will see that it validates for
CSS version 3.

http://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fjeffcarr.info%2Ffiles%2Flinked%2Fw3.org%2Fminwidthtest.css&warning=2&profile=css3&usermedium=all

However, if you attempt to validate the stylesheet when it is assigned a media
type of screen, projection, print, or all three together (but not "all" or
without a media type assigned), you will see the error:

  Line: 8 Context : body

  Invalid number : min-widthParse Error - [empty string] 

http://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fjeffcarr.info%2Ffiles%2Flinked%2Fw3.org%2Fminwidthtest.html&warning=2&profile=css3&usermedium=all

Lines 1-9 of the associated stylesheet are:

body{
font-size:.85em;
font-family:Verdana,"Bitstream Vera Sans",Tahoma,Arial,Helvetica,sans-serif;
color:#000;
background:#fff;
margin:0em;
padding:0em 0em 1em 0em;
min-width:50em;
}