Is type="text/css" necessary in a <link> tag?

It's not required, no.

The part of the HTML Living Standard you're interested in is The link element, which states:

A link element must have either a rel attribute or an itemprop attribute, but not both.

The type attribute gives the MIME type of the linked resource. It is purely advisory. The value must be a valid MIME type string.

For external resource links, the type attribute is used as a hint to user agents...

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.