Link types and profiles

This may be a remedial question but:

Is or isn't it legitimate to add link types (i.e. values for rel
attributes) in document profiles?

http://www.w3.org/TR/html401/types.html#type-links : "Authors may wish to
define additional link types not described in this specification. If they
do so, they should use a profile to cite the conventions used to define the
link types. Please see the profile attribute of the HEAD element for more
details."

but

http://www.w3.org/TR/html401/struct/global.html#profiles : "The profile
attribute of the HEAD specifies the location of a meta data profile." and
goes on to strongly imply that the profile describes how to interpret META
tag data, but is otherwise pretty quiet about how to use it.

There's XMDP, of course, which tries to address link type/meta by
suggesting that you define explicit rel properties - but there's sort of a
semantic leap between <meta name="<property>" value="<value>"> and <link
rel=<value>>

If it is legit, why not legit to add acceptable values for any tag
attribute (without describing *which* tags it's legit on)?

Judson