IndiePreview does one job. You paste a public URL, and it shows the card each platform would draw from that page’s Open Graph and X Card tags: X, LinkedIn, Slack, Discord, and iMessage.
The mockups are drawn from the tags in the fetched HTML. They are not screenshots from the platforms. Every verdict links the platform’s own documentation and shows the date that rule was last checked. Where a platform publishes no exact rule, the page says the mockup is a model rather than a documented fact.
What it will never do: ask for an account, sell a plan, keep a history of what anyone checked, or guess tags that were not in the HTML. If a page adds its tags with JavaScript, or answers a challenge page instead of HTML, IndiePreview says so and stops.