Q:

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

I was wondering whether or not it is necessary to use <link rel="stylesheet" type="text/css" href=...> over <link rel="stylesheet" href=...>.  The rel="stylesheet" marks the information that it is a stylesheet - so text/css doesn't actually add anything as far as I'm concerned. 
The only stylesheet format used by HTML is CSS anyway, so what does text/css 'say' to the browser?  Some websites seem to add the type="text/css" attribute (http://www.jquery.com/), whilst other ones don't (http://www.youtube.com/). 
So, what is the use of type="text/css" in a <link rel="stylesheet"> element, and is it necessary to include it? 

A:

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. 

A:

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...

A:

HTML4.01 does not require the type attribute on a <link> tag.  According to the specification, the type attribute is a hint to the browser.  If the browser does not support the hinted content type, then it can skip fetching it. 

type = content-type [CI]

This attribute gives an advisory hint as to the content type of the content available at the link target address.  It allows user agents to opt to use a fallback mechanism rather than fetch the content if they are advised that they will get content in a content type they do not support. 
Authors who use this attribute take responsibility to manage the risk that it may become inconsistent with the content available at the link target address. 

NOTE: <link> defers to <a>'s definition for the type attribute.