Checkboxes are used for toggling between states. Instead of binding to input.value, we bind to input.checked:
App.svelte
<input type="checkbox" bind:checked={yes}>1
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<script>
let yes = false;
</script>
<label>
<input type="checkbox" checked={yes} />Yes! Send me regular email spam
</label>
{#if yes}<p>
Thank you. We will bombard your inbox and sell
your personal details.
</p>
{:else}<p>
You must opt in to continue. If you're not
paying, you're the product.
</p>
{/if}<button disabled={!yes}>Subscribe</button>