<link rel="stylesheet" type="text/css" media="print" href="print.css">
The HTML4 spec says:
This attribute specifies the intended destination medium for style information. It may be a single media descriptor or a comma-separated list. The default value for this attribute is "screen".
The HTML5 spec says:
The default, if the media attribute is omitted, is "all", meaning that by default links apply to all media.
So if your pages suddenly look a little odd, check that you are declaring a media type for all your style sheets.