<form onsubmit="return false" oninput="o.value = a.valueAsNumber + b.valueAsNumber">
  <label for="a">
    Number one
  </label>
 <input id="a" type="number" step="any"> +
 <label for="b">
    Number two
  </label>
 <input id="b" type="number" step="any">
 <br>
 =
 <output id="o" for="a b"></output>
</form>