How the mockup is built
The card is assembled from the parts people actually recognise: the vote rail down the left with the score between two arrows, the subreddit in bold followed by the muted author and age, a large title, optional body text, and the comment count with a speech bubble underneath. Those elements in that arrangement are what make an image read as Reddit before anyone reads a word of it.
Scores are abbreviated the way Reddit abbreviates them, so 12400 renders as 12.4k. That detail matters more than it sounds: a five-digit integer sitting where a Reddit score belongs is the first thing that makes a mockup look wrong.
The image is drawn on an HTML canvas rather than screenshotted from the page. Text is measured and wrapped to the card width, and the canvas grows to fit however much body text you wrote. It renders at twice the display resolution, so the PNG holds up in a slide or a video thumbnail.
Everything happens in the browser tab. The text is never sent anywhere, the rendering is local, and the download comes out of memory. Nothing needs an account and nothing you type is stored.