RDFa best practices

I have a few questions related to best practices in RDFa mark-up in HTML. I have marked this question as a community wiki, so feel free to edit or add more questions.

  1. When using only one RDF vocabulary in an HTML page (such as the unified namespace used in the design of rNews) is it better to use the vocab attribute to define the default namespace to shorten the mark-up, or, define namespaces using the prefix attribute to achieve a clearer description?
  2. Is it OK to use about="" when referring to the HTML page containing the RDFa mark-up, or, is it better to use explicitly the full URI of the page, such as about="http://example.com/page"? RDFa distillers tend to convert about="" to <>: What happens when the context of the page is lost? <> seems to me like a local identifier, so what happens when you aggregate different distilled RDFa data using about=""? Can you tell the provenance of the <> URIs if you're not using named graphs? I haven't seen RDFa distillers using @base to set the base URI for relative URIs such as <>.
  3. Is there any way how to leverage content from HTML attributes in RDFa? For example, I may want to mark-up the content of an alt attribute of an <img /> as the value for dc:description, or use the value from the title attribute in dc:title. Do I need to duplicate the content of such HTML attributes elsewhere on the page in order to include it in RDFa data?
  1. @prefix and @vocab are not about clarity of description, but about convenience of dealing with long URIs. If you use only a single vocabulary, or one main vocabulary with a few terms from others, then @vocab is appropriate; otherwise you need multiple prefixes anyway, so @prefix is handier.
  2. @about="" is better because it is guaranteed to refer to the current page. It is common on the web to use relative links. This is no different from @href="about.html" in HTML, which is commonplace and also breaks if one isn't careful about relative URIs.
  3. There is no way to include the content of @alt into an RDFa statement.