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
Semantic Interoperability of Metadata and Information in unLike Environments.
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