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.