Hmmm...

Looks like UniversalEditButton may or may not have actually adopted the
rel="edit" form -- http://universaleditbutton.org/Suggestions#Linking_Scheme
lists it and includes a mention of me adding it, but
http://universaleditbutton.org/Add_The_Link still lists the rel="alternate"
format.

This is the kind of thing that's really meant to be open-ended though, I'm a
bit baffled by the idea of rel values being registered. Is that new in
HTML5-land?

Gadget850:

This is the kind of thing that's really meant to be open-ended though, I'm a
bit baffled by the idea of rel values being registered. Is that new in
HTML5-land?

To resolve this, see http://microformats.org/wiki/existing-rel-values#HTML5_link_type_extensions

The description must include:

• "Effect on link"
• "Effect on a and area"
• a link to a spec that documents the keyword as an HTML rel keyword

One the keyword is registered, then the validator will pick it up and no longer generate this annoying error.