Hey I just released a library called embedz specifically for this, it handles everything for you, and is about 5x faster than a regular youtube embed

You can check it out here, GitHub

To use it install the npm package

npm i @embedz/svelte

Then import the library in to your svelte app

<script>
  import { YouTube, Vimeo } from "@embedz/svelte";
</script>

<YouTube 
  id="KRVnaN29GvM" 
  posterquality="max"
/>