In the DOM, everything is a string. That's unhelpful when you're dealing with numeric inputs — type="number" and type="range" — as it means you have to remember to coerce input.value before using it.
With bind:value, Svelte takes care of it for you:
App.svelte
<label>
<input type="number" bind:value={a} min="0" max="10" />
<input type="range" bind:value={a} min="0" max="10" />
</label>
<label>
<input type="number" bind:value={b} min="0" max="10" />
<input type="range" bind:value={b} min="0" max="10" />
</label>1
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<script>
let a = 1;
let b = 2;
</script>
<label>
<input type="number" value={a} min="0" max="10" /> <input type="range" value={a} min="0" max="10" /></label>
<label>
<input type="number" value={b} min="0" max="10" /> <input type="range" value={b} min="0" max="10" /></label>
<p>{a} + {b} = {a + b}</p>