Twitter card validator
X’s own Card Validator answers a request by redirecting to a sign-in page, and the pages that still rank for it link straight to it. This reads the tags you paste and tells you what a card would carry, without an account.
Your markup stays in this browser tab. Nothing is fetched and nothing is stored.
summary is the default, which most checklists miss
A page with no twitter:card tag is not a page with no card. The type defaults to summary, so a title, a description and an image still produce one. What you lose by not declaring it is the large image, not the card.
That is why this reports a missing tag as something worth knowing rather than something broken, and why it separates the two lists. A tool that calls everything an error teaches you to ignore all of it.
What this does not do
It does not draw a preview. Nobody outside X can say what X will render from a set of tags, and a picture that claims to is a guess at a renderer it cannot see. A test holds this tool to that, so adding a preview later means arguing with the test first.
It does not fetch your page either. You paste the head, because a tool that takes a URL and fetches it from our servers is a tool that can be pointed at addresses inside our own network.
And it cannot tell you whether X has cached an older version of your card. Nothing can, since the tool that used to say so is the one that went away.