Attributes rel and rev now have defined values:

alternate, appendix, bookmark, cite

chapter, contents, copyright, first

glossary, help

icon

Refers to a resource that represents an icon.

index, last, license

meta

Refers to a resource that provides metadata, for instance in RDF.

next

p3pv1

Refers to a P3P Policy Reference File

prev

role

Indicates the purpose of the resource.

section, stylesheet, subsection, start, top

up

Refers to the resource above in a hierarchically structured set