svelte/test/runtime/samples/transition-css-duration/main.html

{{#if visible}}
	<div transition:foo></div>
{{/if}}

<script>
	export default {
		transitions: {
			foo() {
				return {
					duration: 100,
					css: t => {
						return `opacity: ${t}`;
					}
				};
			}
		}
	};
</script>