It's actually IE7 and IE8, and only for CSS file URLs (as opposed to URLs in CSS files). Details of protocol-relative URLs here:

http://blog.httpwatch.com/2010/02/10/using-protocol-relative...

...and the unfortunately double-loading bug for CSS files in IE7/8 here:

http://www.stevesouders.com/blog/2010/02/10/5a-missing-schem...