It’s not required with the HTML5 spec, but for older versions of HTML is it required.
Html 4 W3.org spec
http://www.w3.org/TR/html40/struct/links.html#edef-LINK
http://www.w3.org/TR/html40/present/styles.html
Type stands for The MIME type of the style sheet. The only supported value I have ever seen is Text/CSS, which is probably why HTML5 has dropped it. I imagine they had it for earlier versions to allow future expansion possibilities which never happened.
Using HTML5 and not specifying the type, I have run so far into no problems with compatibility even when testing older versions of IE.