This is a SvelteKit app. You can make your own by typing the following into your command line and following the prompts:
npm create svelte@latest
The page you're looking at is purely static HTML, with no client-side interactivity needed. Because of that, we don't need to load any JavaScript. Try viewing the page's source, or opening the devtools network panel and reloading.
The Sverdle page illustrates SvelteKit's data loading and form handling. Try using it with JavaScript disabled!