svelte/test/runtime/samples/transition-js-nested-intro/Child.html

<div transition:foo><slot></slot></div>

<script>
	export default {
		transitions: {
			foo: function ( node, params ) {
				return {
					delay: 50,
					duration: 100,
					tick: t => {
						node.foo = t;
					}
				};
			}
		}
	};
</script>