A prototype of how RDFa annotations can be done on XHTML declaratively using client-side XSLT. Double-click on some text, enter any name in the prompt and inspect the DOM of the bold <span>. You should see RDFa attributes such as about, content, and property.
Works at least in Firefox.
Uses Saxon-CE 1.1 as XSLT 2.0 processor in the browser: http://www.saxonica.com/ce/index.xml
An updated version is also available as Saxon-JS (though not open-source): http://www.saxonica.com/saxon-js/index.xml