<a> has these proposed attributes:
Proposed - Level 1. An attribute
rel may give the
relationship (s) described by the hypertext link. The
value is a comma-separated list of relationship values.
Values and their semantics will be registered by the
HTML registration authority. The default relationship if
none other is given is void.
rel should not be present
href is present.
Proposed - Level 1. The
rev attribute is the same as
the semantics of the link type are in the reverse
direction. A link from A to B with rel="X" expresses the
same relationship as a link from B to A with rev="X". An
anchor may have both