rel=tooltip on link (a) tags not HTML5-valid?

I’ve been using your (fantastic) tooltips on links like this:

<a rel="tooltip" href="target_url" title="tooltip text">link title</a>

However, this doesn’t validate in HTML5 format. validator.w3.org gives:

Bad value tooltip for attribute rel on element a: The string tooltip is not a registered keyword or absolute URL.

Any ideas?

http://wordpress.org/plugins/responsive-mobile-friendly-tooltip/