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 are, just as importantly, defined by what they are not: not a new language; not infinitely extensible and open-ended; not an attempt to get everyone to change their behavior and rewrite their tools; not a whole new approach that throw