RDF-3T is a metaformat(a language used to describe another language) that uses machine tags to describe RDF triples in XHTML. A Machine Tag is a word first used by flicr as a technical term for a triple tag. Machine Tags are in the wild are just like normal tags but have a special syntax that allows the user to express further semantics(meaning) in a tag and is made up of a namespace, predicate and a value, for example you could tag an image on flicr...
The namespace is "lastfm" the predicate is an "event" and the value is "887898".
The first things I thought when I first saw machine tags was hey that looks a lot like a RDF statement and I wonder if you could use the Machine Tag syntax to describe a RDF triple.
The first problem was that to express all the information contained in a RDF Triple in a tag was just too much so I though to use the @class area instead. A html class can be used on almost any html element and a class accepts data values where as a tag is just text. RDF-3T does not use rel=tag at all so it wont conflict with any existing usage of machine tags.
In RDF-3T namespaces are declared using meta tags in the head of a page, and then referenced in a class using any of the terms(predicates) declared in the namespace document itself. Values are slightly different as RDF-3T uses keywords or pointerers to tell the machine where the value is, values can also be a string such as an ISO8601 formatted date or a geo location.
RDF-3T uses the @id attribute to declare what a thing is about, The id attribute can contain a plain text string, which is used as a relative uri reference, or the id can be a compact uri expression(curie) for example the curie expression ...
... in RDF-3T would resolve to http://weborganics.co.uk/#weborganics.
RDF-3T triples are typically extracted using a GRDDL profile available at http://weborganics.co.uk/Profiles/RDF-3T, you can view the output using the W3C GRDDL service, for example here: RDF-3T to RDF.
A more in depth explanation of RDF-3T along with a few tests and example mark-up can be found at http://weborganics.co.uk/RDF-3T/. Any feedback on the RDF-3T syntax or anything you are not happy with is welcome by either email or simply by adding a comment to this page.