question.svelte

<script>
	
	import {onMount} from 'svelte'
	
	let name = 'world';
	let foo;
	let cssClass = "test"
	
	onMount(()=>{
		foo.addEventListener('click', function(){
				cssClass = "animate"
		})
	})
</script>

<h1 bind:this={foo} class={cssClass}>Hello {name}!</h1>

<style>
	.test{
		font-size: small;
	}
	
	.animate{
		font-size: large;
		transition: font-size .3s;
	}
</style>