What if you are transforming to html from a more semantic markup language like TEI? In this circumstance, isn’t HTML your presentation layer? How concerned with semantics should you be at this stage, when you are trying to prepare the material for human usage? Well, it is still easier to maintain CSS files than changing inline styles, font tags and misused tags in your XSL files. And you still get the accessibility benefits of using semantic html.
But should you care about machine readability if you are also publishing the more-semantic TEI source? Should you bother about putting RDFa into your html? Well, arguably you should. As I see it, TEI is for describing the elements of a document; transforming to HTML with RDFa can add a meaningful interpretative layer, stating what the document is saying about the external world. In addition, the semantics of RDF and HTML microformats are more widely understood by machines than the semantics of TEI.
But what about if you are providing the information in a (linked) RDF document as well? Is it still a beneficial thing to add these extra semantics to your html document? After all, why give machines the trouble of extracting RDF from your html if you are giving it to them pure and free anyway? Are, or will there be, user agents (for example, browser plugins, screenreaders) that will find these inline semantic statements more useful than pure RDF in a separate document?