I found out a few things about that Neocities thingie that takes screenshots of every page you update. For starters, it’s buggy and might render noscript elements as visible text.

But here’s something more interesting about it: It has its own user agent, Screenjesus. As other people has reported before, this makes it possible to change your site dynamically so the screenshots that it grabs are different to what your visitors will see visiting your page:

if (navigator.userAgent === "Screenjesus") {
  // change something,
  // e.g. replace the body for an image,
  // swap your theme or generate a
  // cool looking thumbnail
}

I didn’t do anything too interesting with this information, but now I’m forcing Screenjesus to use my 2000s theme, so that’s what the thumbnails of my website are showing.