HTML and CSS Reference
Microdata and Custom Data
One of the primary benefits of web standards, promoted for over 10 years by standards
evangelists, was that more semantic markup would provide greater context and
meaning to content.
By the mid 2000s, web standards were catching on, but the “standardistas” recognized
that semantic markup alone was not enough to convey all meaning related to
content—for example, that some content is personal data or that links may represent
To solve this problem, first came XFN (XHTML Friends Network, see http://gmpg.org/
relied on the class and rel attributes—both valid (X)HTML—but, because those
attributes had more appropriate uses, some web professionals felt that using them to
convey additional semantics was somewhat of a hack.
At about the same time, the W3C XHTML 2 Working Group created the RDFa
specification, based on the Resource Description Framework, to enable reuse of
existing XHTML attributes—and the addition of some new ones—to support
structured machine-readable data.
Microformats, in particular, caught on quickly with web professionals. Web builders
started using them to mark up personal information, resumes, event data, and even
cooking recipes. Search engines picked up on this and began parsing the additional
data found in microformats and RDFa to improve search results.
With HTML5, the web community has a renewed focus on strictly adhering to the
specification. Rather than continuing to extend—some would say abuse—the class
and rel attributes, HTML5 (at least via the WHATWG spec) introduces microdata ,
which gives us a new syntax for marking up these additional structured semantics.