svelte/documentation/examples/09-transitions/00-transition/App.svelte

<script>
	import { fade } from 'svelte/transition';
	let visible = true;
</script>

<label>
	<input type="checkbox" bind:checked={visible} />
	visible
</label>

{#if visible}
	<p transition:fade>Fades in and out</p>
{/if}