When you define a rel link type, you automatically define it for rev too. From W3C:
Consider two documents A and B.
Document A: <LINK href="docB" rel="foo">
Has exactly the same meaning as:
Document B: <LINK href="docA" rev="foo">
So when I tag this entry with Tags,
Technorati could use
rev="tag" on the link back.
And I think they should. Because the
Tags page contains many links.
Some are links to pages with the Tags tag,
others are not. A human can see the difference from the layout, but robots don't read layout.
It would be useful for a search robot to know that all links with
have something in common with each other, and with the last segment of the page's URL.
At Technorati, anyone can get a link back when they tag their own page.
At sites like http://hoppa.com/, anyone could also tag their page as
But that will not give an automatic link back, because Hoppa is edited.
In such cases,
rev="tag" would be even more valuable to search engines.
But to be tag compatible, the URL should probably have been
Still, Hoppa is better off than DMOZ, where the page that is listing
weblog search engines
has the tag
Search_Engines, which is far more general than the content of the page.