If you’re using fancybox and tried to validate your code you’ll get this error if you use the rel attribute: Bad value fancybox for attribute rel on element a: The string fancybox is not a registered keyword.

Luckily, there’s an easy fix: just replace the rel attribute with the data-fancybox-group attribute and it will work in the exact same way and validate!

Thanks to JFK for this easy fix.