This is sometimes called a forward link. Although the HREF HTML tag attribute is normally the only means used to identify a target document, the browser is permitted to use the REL HTML tag attribute to decide whether to use the HREF value or how it should be used.
The following HTML version 4.0 standard link types are permitted in this property:
MSIE adds these as well:
When used or tested within a script, any comparisons should be case insensitive.