`twitter.card`

Default: warn
Plugin: twitter

A page has a valid twitter:card.

Why: X, formerly Twitter, uses twitter:card to choose the layout of a link preview. Without a valid value, it shows a plain link. The valid values are summary, summary_large_image, app, and player.

When off: pages without, or with an invalid, twitter:card pass.

<!-- ✗ Fails -->
<meta name="twitter:card" content="big" />

<!-- ✓ Passes -->
<meta name="twitter:card" content="summary_large_image" />

References

X: Cards markup: twitter:card sets the card type, from a fixed list.
Patrick Stox: Twitter cards: X falls back to the Open Graph tags for the title, description, and image, but not for the card type. The rule warns.