The correct approach to markup "keywords" of a blog post by using HTML5 and Microdata?

html, microdata, schema.org, semantic-markup

Solution

Update: The definition of Schema.org’s `keywords` property changed. Now it makes clear that it expects multiple tags, and that they should typically be comma-separated.

tl;dr: If you want to use a `ul` element for your tags, using Schema.org’s `keywords` property could look like this in Microdata:

<article itemscope itemtype="http://schema.org/BlogPosting">

  <footer>
    <ul itemprop="keywords">
      <li><a href="/tags/foo" rel="tag">foo</a>,</li>
      <li><a href="/tags/bar" rel="tag">bar</a></li>
    </ul>
  </footer>

</article>

If using it like that, you have to make sure that the `ul` contains no other non-tag text.

HTML

The `meta` element with the `name` value `keywords` offers a way to specify keywords that apply to the whole page. These keywords consist of text only, so you can’t use URIs.

So for the typical blog tags, you’d go the following way (possibly in addition to `meta`-`keywords`).

Link type

HTML5 defines the link type `tag`:

The `tag` keyword indicates that the tag that the referenced document represents applies to the current document.

Note that this link type can only be used on single post pages as the tag always applies to the whole page.

(Attention: `tag` is a Microformat, too, but it has a different definition: You may only use specially crafted URLs for the Microformat `tag`.)

If your tags are more like categories (controlled vocabulary instead of free tagging), you could use the link type `category` (possibly in combination with `tag`).

Markup for the tags

You might use a `ul` or a `dl`:

<ul>
  <li><a href="/tags/foo" rel="tag">foo</a></li>
  <li><a href="/tags/bar" rel="tag">bar</a></li>
</ul>

<dl>
  <dt>Tags</dt>
  <dd><a href="/tags/foo" rel="tag">foo</a></dd>
  <dd><a href="/tags/bar" rel="tag">bar</a></dd>
</dl>

I’d go with the `dl` when you also have other metadata to list, e.g. the author, publication date, etc.

A `div` with comma-separated `a` elements would work also, of course:

<div>Tags: <a href="/tags/foo" rel="tag">foo</a>, <a href="/tags/bar" rel="tag">bar</a></div>

Container for the list

The list should be part of a `footer` element (inside the `article` if you use one for your blog post):

A footer typically contains information about its section such as who wrote it, links to related documents, copyright data, and the like.

Vocabularies

Schema.org

`CreativeWork` defines the property `keywords` (`BlogPosting` inherits it):

Keywords or tags used to describe this content. Multiple entries in a keywords list are typically delimited by commas.

It expects plain text, so there’s no way to provide the tag URLs in Schema.org.

As `keywords` expects a comma-separated list of tags, `dl` can’t be used (unless the `dl` contains nothing else than tags).

With an `ul`, it could look like this in Microdata:

<ul itemprop="keywords">
  <li><a href="/tags/foo" rel="tag">foo</a>,</li>
  <li><a href="/tags/bar" rel="tag">bar</a></li>
</ul>

If using a `div`, you’d need to add another `div`/`child` so that the label "Tags:" gets not interpreted as tag itself:

<div>Tags: <span itemprop="keywords"><a href="/tags/foo" rel="tag">foo</a>, <a href="/tags/bar" rel="tag">bar</a></span></div>

Common Tag

Common Tag is a (RDF) vocabulary for tagging. In contrast to Schema.org’s `keywords` property it uses URIs for tags, not text only.

Example from their Quick Start Guide in RDFa:

<div xmlns:ctag="http://commontag.org/ns#" rel="ctag:tagged">
   NASA's <a typeof="ctag:Tag" 
             rel="ctag:means"
             href="http://rdf.freebase.com/ns/en.phoenix_mars_mission"
             property="ctag:label">Phoenix Mars Lander</a> has deployed its robotic arm. 
</div>

Explanation: The content is tagged (`ctag:tagged`) with a tag (`ctag:Tag`). This tag is defined (`ctag:means`) by the URI "http://rdf.freebase.com/ns/en.phoenix_mars_mission" and it’s called (`ctag:label`) "Phoenix Mars Lander"

Instead of `ctag:Tag` you could use the class `ctag:AuthorTag` (which means that it’s tagged by the content author).

MOAT

There is also the MOAT vocabulary, which stands for "Meaning Of A Tag". Unfortunately, their website seems to be gone (?).

An ontology that let users define relationships between Tag objects and URIs of Semantic Web resources

Problem

The `keywords` property from Schema.org, as it being in plural form suggests, seems to imply its value should be a line of comma-separated text. Like the following example: ``` <p itemprop="keywords">lorem, ipsum, dolor, sit, amet</p> ``` That reminds me of the `<meta name="keywords" content="lorem, ipsum, dolor, sit, amet">` tag we used to put within `<head></head>` for SEO reason. However, from semantics' perspective, I think the above example is incorrect because keywords should be a list of words. Thus it should be markuped by using the `<ul>` element. Is there a common consensus on how the keywords of a blog post should be markuped by using HTML5 and Microdata?

Original source