HTML5
HTML
<form onsubmit="return false" oninput="o.value = parseInt(a.value) + parseInt(b.value)">
<input name="a" type="number"> +
<input name="b" type="number"> =
<output name="o" ></output>
</form>
<form onsubmit="return false" oninput="o.value = parseInt(a.value) + parseInt(b.value)">
<input name="a" type="number"> +
<input name="b" type="number"> =
<output name="o" ></output>
</form>