Svelte version 3 used to statically analyze at compile time which things could change which things and it would build a "direct" reactivity system from that. So kinda both compile time and runtime.

But Svelte 5 changed to use a signals based approach just like Vue and Solidjs, which is runtime based and more flexible. Svelte chose to add some compile time syntax sprinkles on top that they call "runes", because they like a simpler syntax. But signals are fundamentally a runtime concept.