<form oninput="output.value=parseInt(nb1.value)+parseInt(nb2.value)">
  <input type="number" id="nb1" value="6">
    +
  <input type="number" id="nb2" value="6">
    =
  <output name="output" for="nb1 nb2"></output>
</form>

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.