How to enable tag name twitter:domain for twitter cards in w3c validator?
<meta name="twitter:domain" content="...." >
How to enable tag name twitter:domain for twitter cards in w3c validator?
<meta name="twitter:domain" content="...." >
If you are using HTML5, you may only use values listed in the specification and in the WHATWG wiki page "MetaExtensions" (see also this answer).
As twitter:domain is registered in the wiki, it’s valid to use it.
So the validator is not up to date.