Reference: *Style Sheet Module at the W3C
The Style Sheet Module declares a single element,
style, which may be included
in an XHTML 1.1 document only as a child of the head element and which
encloses a set of document-specific styles. These styles are typically CSS, specified with
a type attribute value of "text/css".
To associate an external style sheet file with the document, use the link element,
from the Link Module.