Remove datatype attribute from meta about tag
Apparently the meta tag isn't the problem. The problem is the validation service and how you use it. After some digging I found this article. The author suggest using a different validation type namely: http://validator.w3.org/nu/ with the following preset set in the additional options: "HTML5 + SVG 1.1 + MathML 2.0 + RDFa 1.1 + Microdata". This will validate said meta tags without any errors.