Question

My understanding is that mime types are set by the web server. Why do we add the type="text/javascript or type="text/css" attribute? Isn’t this a useless and ignored attribute?

Answer 1

Douglas Crockford says:

type="text/javascript"

This attribute is optional. Since
Netscape 2, the default programming
language in all browsers has been
JavaScript. In XHTML, this attribute
is required and unnecessary. In HTML,
it is better to leave it out. The
browser knows what to do.

He also says:

W3C did not adopt the language
attribute, favoring instead a type
attribute which takes a MIME type.
Unfortunately, the MIME type was not
standardized, so it is sometimes
"text/javascript" or
"application/ecmascript" or something
else. Fortunately, all browsers will
always choose JavaScript as the
default programming language, so it is
always best to simply write <script>.
It is smallest, and it works on the
most browsers.

For entertainment purposes only, I tried out the following five scripts

  <script type="application/ecmascript">alert("1");</script>
  <script type="text/javascript">alert("2");</script>
  <script type="baloney">alert("3");</script>
  <script type="">alert("4");</script>
  <script >alert("5");</script>

On Chrome, all but script 3 (type="baloney") worked. IE8 did not run script 1 (type="application/ecmascript") or script 3. Based on my non-extensive sample of two browsers, it looks like you can safely ignore the type attribute, but that it you use it you better use a legal (browser dependent) value.