Property $class

App.xht

<script>
    import Child from './Child.xht';
    let cond = true;
</script>

<input type="checkbox" :checked={cond} />

<Child class="red italic" class:bold={cond} />

<style>
    .red {color: red;}
    .italic {font-style: italic;}
    .bold {font-weight: bold;}
</style>

Child.xht

<div class={$class}>
    Child
</div>