The REL attribute is used to define the relationship between
the linked file and the HTML document.
REL=StyleSheet specifies a persistent or preferred style while
REL="Alternate StyleSheet" defines an alternate style.
A persistent style is one that is always applied when style sheets are enabled. The absence of the TITLE attribute, as in the first tag in the example, defines a persistent style.
A preferred style is one that is automatically applied,
such as in the second tag in the example.
The combination of REL=StyleSheet and a TITLE attribute specifies a
preferred style. Authors cannot specify more than one preferred style.
An alternate style is indicated by REL="Alternate StyleSheet".
The third tag in the example defines an alternate style,
which the user could choose to replace the preferred style sheet.