Svelte Guideline: Logic — Use {#await} for promises. Description: Handle async states. Do: {#await} for loading/error states. Don't: Manual promise handling. Good Example: {#await promise}...{:then}...{:catch}. Bad Example: {#if loading}...{#if error}. Severity: Medium. Docs: https://svelte.dev/docs/logic-blocks#await.