There is a demo shows you the minimal code and working example of how to set up a astro project to make use of Svelte compiles to Custom Element feature at https://stackblitz.com/edit/svelte-ce-on-astro-demo?file=README.md.
- Svelte allow you to compile its component to a custom element (aka Web Component)
- Astro doesn't have a guide on how to do this but I figured out how to and thus the sharing