If I have a page with

<meta name="viewport" content="width=device-width,minimum-scale=1">
<meta name="anything" value="this,that,theother">

The validator throws

validator.js:274 http://localhost:8000/templates/amptemplate.html:11:4 The attribute 'name' in tag 'meta name=viewport' is set to the invalid value 'anything'. (see https://github.com/ampproject/amphtml/blob/master/spec/amp-html-format.md#vprt)

<meta name="twitter:card" content="summary"> is however accepted.

Bug or are you blocking some meta tags intentionally? If so where can I find the list of approved meta tags?