HTML 5 data- Attributes: HTML and XML begin to merge.
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).
So, you could have:
<div data-firstname="Deane" data-lastname="Barker>
Of course, you can do this now. You’ll just validate in HTML 5.