Link is allowed in BODY.
I had same problem validating link tag in HTML5 and I solved it with this
<link rel="stylesheet" property="stylesheet" href="https://stackoverflow.com/questions/6236097/css/homepage.css">
Need to have both property and rel tag
UPDATE 2016 (thanks to yuyokk below):
There was a change to HTML5 spec recently that allows having links in the body