<form onsubmit="return false" oninput="output.value = parseInt(inputOne.value) + parseInt(inputTwo.value)">
  <input name="inputOne" type="number" value="2"> +
  <input name="inputTwo" type="number" value="3"> =
  <output name="output">5</output>
</form>

body {
  font-family: Roboto,"Helvetica Neue",Arial,sans-serif;
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
}
a {
  font-size: 2rem;
  text-decoration: none;
  color: teal;
}