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
N. Nerkar, N. Jain, D. Mehta, K. Sodha, und C. Barot. International Journal on Recent and Innovation Trends in Computing and Communication, 3 (2):
542--544(Februar 2015)