About @rel='tag' HTML5 conformity

[WraithKenny]

The comments on #17632 have prompted concern over the use of rel='tag' on HTML5 documents containing multiple blog posts (from different taxonomies) and whether that's valid or not. A conditional, or parameter, to filter out (in the get_the_category_list function) the rel attribute for pages containing posts from multiple taxonomies would address this issue at least until, if and when, the specs can be clarified or adjusted. See #17632 for a much more detailed discussion.

[WraithKenny]

Seems to me new filters are the best answer.

[Marventus]

I checked the output of the get_the_category_list function in home, page, post, category archive, date archive, and author archive, and this is what I found:
1. Home - Categories of the last post being retrieved (in my case, the oldest);
2. Page - empty;
3. Post - Post Categories;
4. Category Archive - Archive Category;
5. Date Archive - Idem 1. Home;
6. Author Archive - Idem 1. Home.
So, it seems output is correct for cases 2-4 and incorrect for 1,5-6. This could be easily fixed with Conditional Tags or, as @WraithKenny suggested, with filters.

[pbiron]

I posed a question on the ​HTML5 comments mailing list asking about 1) whether @rel='tag' is appropriate for controlled vocabularies and 2) what is the scope of @rel='tag'. We'll see what the response is.

[WraithKenny]

After letting this sit for a few months, I'd like to summarize where I'm at at least.

I think having taxonomies tagged on list/archive pages is fine, assuming that those tags apply to the section of the content they are included into, which as being part of the page, can be correctly considered valid, as the page is in some part about that content. ​http://microformats.org/wiki/rel-tag-faq#What_about_Scope.3F Below is my interpretation:

1. Where the rel=tag is apporpriate: When the content (or some section of the content) is in the taxonomy (tag or category) being linked to. This includes the individual listed posts in category and tag archive pages. Collection of tags here are OK (not a "Tag Cloud" as mentioned in the microformats).

2. Where it's not appropriate: arbitrary links to Category or Tag Archives (like from the sidebar, Tag Cloud Widget). The fact that it links to the tag's page is irrelevant. These links shouldn't include rel="tag"

rel="tag" is NOT designed for "tagging" arbitrary URLs or external content. ​http://microformats.org/wiki/rel-tag

I think that using get_the_category_list in the loop is as valid (as far as outputting rel="tag") as using it on the single post view.