<form onsubmit="return false" oninput="output.value = price.valueAsNumber + (price.valueAsNumber * vat.valueAsNumber)">
  <input name="price" type="number" value="20"> +
  <input name="vat" type="number" value="0.2" step="0.1" max="1"> =
  <output name="output">24</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;
}