In order for the style sheets to influence the presentation, the UA must be aware of their existence. The HTML specification define how to link HTML with style sheets. This section is therefore informative, but not normative:
<HTML>
<HEAD>
<TITLE>title</TITLE>
<LINK REL=STYLESHEET TYPE="text/css"
HREF="http://style.com/cool" TITLE="Cool">
<STYLE TYPE="text/css">
@import url(http://style.com/basic);
H1 { color: blue }
</STYLE>
</HEAD>
<BODY>
<H1>Headline is blue</H1>
<P STYLE="color: green">While the paragraph is green.
</BODY>
</HTML>
The example shows four ways to combine style and HTML: using the 'LINK' element to link an external style sheet, a 'STYLE' element inside the 'HEAD' element, an imported style sheet using the CSS '@import' notation, and a 'STYLE' attribute on an element inside 'BODY'. The latter option mixes style with content and loses the corresponding advantages of traditional style sheets.
The 'LINK' element references alternative style sheets that the reader can select, while imported style sheets are automatically merged with the rest of the style sheet.
Traditionally, UAs have silently ignored unknown tags. As as result, old UAs will ignore the 'STYLE' element, but its content will be treated as part of the document body, and rendered as such. During a transition phase, 'STYLE' element content may be hidden using SGML comments:
<STYLE TYPE="text/css"><!--
H1 { color: green }
--></STYLE>
Since the 'STYLE' element is declared as "CDATA" in the DTD, conforming SGML parsers will not consider the above style sheet to be a comment that is to be removed.