Meta tag twitter:domain not recognized

When checking URL younique-xperience.nl the following error is shown:

 

Line 68, Column 59: Bad value twitter:domain for attribute name on element
meta: Keyword twitter:domain is not registered. 

        <meta name="twitter:domain" content="Younique Xperience"/>

Syntax of metadata name:

A metadata name listed in the HTML specification
<http://www.whatwg.org/specs/web-apps/current-work/multipage/semantics.html#
standard-metadata-names>  or listed in the WHATWG wiki
<http://wiki.whatwg.org/wiki/MetaExtensions> . You can register metadata
names on the WHATWG wiki <http://wiki.whatwg.org/wiki/MetaExtensions>
yourself. 

 

 

The meta keyword twitter:domain is valid (listed and approved in WHATWG
wiki), but not recognized by the W3C Validator yet. Please fix.

 

Best regards,

Ruben van Doorn