Simply, the specification for custom data attributes states that any attribute that starts with "data-" will be treated as a storage area for private data (private in the sense that the end user can't see it - it doesn't affect layout or presentation).
This allows you to write valid HTML markup (passing an HTML 5 validator) while, simultaneously, embedding data within your page
Microformats: Recognizing the complexity of RDF and OWL, the goal of microformats is to embed basic semantics (meaningful relationships) right into HTML pages. It is not as expressive right now as RDF and OWL, but it's compact & and uses at-hand XHTML. A
Most of the tag clouds I have marked up have had over 10 variations in size or gradients of popularity/importance. This makes it virtually impossible to find standard elements to mark up these gradients and still give them meaning when styles are not a
What are microformats?
Designed for humans first and machines second, microformats are a set of simple, open data formats built upon existing and widely adopted standards.
R. Khare, and T. Çelik. WWW '06: Proceedings of the 15th international conference on World Wide Web, page 865-866. New York, NY, USA, ACM Press, (2006)
R. Khare, and T. Celik. WWW '06: Proceedings of the 15th international conference on World Wide Web, page 865--866. New York, NY, USA, ACM Press, (2006)