category
and tag
are valid values. Some validators are not up to date.
tag
is defined in the HTML5 spec.
category
is listed at http://microformats.org/wiki/existing-rel-values#HTML5_link_type_extensions, which is the normative registry for link type extensions.
If both link types apply, it's correct to separate them with whitespace.
Note: don't use these link types on any link to the category/tag page; only use it where the linking page is categorized/tagged with the corresponding category/tag. So a page listing all categories shouldn't use the rel
value "category
" for the links.