Hi Marventus,
Do you have information that indicates “category” will be added to the legal rel values in html5 specification? I have never seen it talked about in any documentation or white paper. I don’t believe its appearance in WordPress will be relevant in it becoming a part of the html5 standard. So I’m not sure why it’s there in the first place and I’m not sure what value it has to wait until html 5 revision is official. It’s invalid now and it has every likelihood of being just as invalid as when html5 is the new official revision.
The link I posted shows a table of available rel values and their valid use in relation to link, a, and area. link is not an attribute it’s a tag name like a. a is an anchor, like for creating hyperlinks, link is for bringing in external assets like stylesheets. rel is an attribute that’s available on both link and a, and area. The “link type” is exactly what “rel” is.