Svelte Runes
Svelte 5's new reactivity (October 2024) — replaces the let x = 0 magic with explicit runes: $state(0), $derived(...), $effect(...).
Rich Harris introduced after the old "auto-reactivity" didn't scale to large components. $-syntax is still compile-time magic but now easy to reason about. Controversial: broke Svelte 4-to-5 migration. Includes Snippets (declarative render functions) that replace slots.