GRDDL (pronounced 'griddle') is a markup format for Gleaning Resource Descriptions from Dialects of Languages. It is a W3C Recommendation, and enables users to obtain RDF triples out of XML documents, including XHTML. The GRDDL specification shows examples using XSLT, however it was intended to be abstract enough to allow for other implementations as well. It became a Recommendation on September 11, 2007. 
How it works
XHTML and transformations
A document specifies associated transformations, using one of a number of ways.
For instance, an XHTML document may contain the following markup:
profile= "http://www.w3.org/2003/g/data-view http://dublincore.org/documents/dcq-html/ http://gmpg.org/xfn/11" > rel= "transformation" href= "grokXFN.xsl" />
Document consumers are informed that there are GRDDL transformations available in this page, by including the following in the
profile attribute of the
The available transformations are revealed through one or more
rel= "transformation" href= "grokXFN.xsl" />
This code is valid for
XHTML 1.x only. The
profile attribute has been dropped in HTML5, including its XML serialisation.
Microformats and profile transformations
If an XHTML page contains Microformats, there is usually a specific profile.
For instance, a document with hcard information should have:
profile= "http://www.w3.org/2003/g/data-view http://www.w3.org/2006/03/hcard" >
profile= "http://www.w3.org/2003/g/data-view" >
Use of this profile licenses RDF data extracted by rel= "profileTransformation" href= "../vcard/hcard2rdf.xsl" >hcard2rdf.xsl from href= "http://www.w3.org/2006/vcard/ns" >the 2006 vCard /RDF work .
The GRDDL aware agent can then use that profileTransformation to extract
all hcard data from pages that reference that link.
XML and transformations
In a similar fashion to XHTML, GRDDL transformations can be attached to XML documents.
XML namespace transformations
Just like a profileTransformation, an XML namespace can have a transformation associated with it.
This allows entire XML dialects (for instance, KML or Atom) to provide meaningful RDF.
An XML document simply points to a namespace
xmlns= "http://example.com/1.0/" >
and when fetched,
http://example.com/1.0/ points to a namespaceTransformation.
This also allows very large amounts of the existing XML data in the wild to become RDF/XML with minimal effort from the namespace author.
Once a document has been transformed, there is an
RDF representation of that data.
This output is generally put into a database and queried via SPARQL.
GRDDL consumers (also known as GRDDL aware agents)
microformats, a simplified approach to semantically annotate data in websites
RDFa, a W3C Recommendation for annotating websites with RDF data
eRDF, an alternative to RDFa
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