X removed the preview from its official Card Validator around 2022 when Twitter rebranded to X; the old cards-dev.twitter.com/validator URL no longer renders a preview. The only way to see the real, live card today is to compose a post (or a draft) on X itself, or to share the link. This Twitter Card Validator fills the gap by parsing the twitter:* and og: tags you paste and rendering an X-style card preview by card type, plus a full lint — but it is explicit that it is an APPROXIMATION of how X renders, not X’s actual renderer. It runs entirely in your browser, so unlike a server-side validator it never uploads or caches your page, which makes it safe for staging or unreleased pages; after you publish, confirm the real card by posting on X.

twitter:card is the only strictly required Twitter tag, and its value must be summary, summary_large_image, app or player — use summary_large_image for the big-image preview most sites want. twitter:title, twitter:description and twitter:image refine the card, and twitter:site / twitter:creator add the @handles of the site and author (they must start with @). Crucially, X falls back to your Open Graph tags: if twitter:title, twitter:description or twitter:image are absent, X reads og:title, og:description and og:image instead. So in practice a complete set of og: tags plus a single twitter:card tag produces a correct card. This validator lints the twitter:* tags AND shows every place where X will fall back to og:, and previews the card the way X renders it for the chosen card type.

The image must be an ABSOLUTE https URL — X cannot fetch a relative or http path — and stay under 5 MB, in JPG, PNG, WEBP or GIF (avoid an animated GIF on the large card). The size depends on the card type: a summary_large_image card crops to a 1.91:1 ratio, so use about 1200×628 (the documented minimum is 300×157 and the maximum dimension is 4096×4096), while a summary card uses a square ~1:1 image of at least 144×144. This validator flags a missing, relative or http image, shows the exact spec table for the chosen card type, and confirms when the image is an absolute https URL — and because X falls back to og:image, it tells you when og:image is covering for a missing twitter:image.

The Twitter Card Validator is 100% client-side. The page source (or twitter:* form) you paste is parsed locally with the browser’s DOMParser, and the preview and validation are built on your device — nothing is fetched, uploaded, logged or stored on a server, and no URL is crawled. That makes it safe for staging, unreleased or internal pages you do not want to expose, unlike online card validators that fetch your page and cache it on their servers. Because the browser cannot fetch most live URLs directly (cross-origin / CORS) and the tool uses no server proxy, you paste your source rather than a URL. It works offline once cached as a PWA, supports dark mode and mobile with a sticky action bar, and closing the tab discards everything you entered. It shares the same socialMetaEngine as the sibling Open Graph Checker, so both tools parse and validate from one source of truth.