What HTML semantics should I use to make the output of a custom calculator accessible?

I’m developing a simple calculator tool. Right now, I’m using an <input> element with the readonly attribute to style the result of the calculation. However, while I think that this produces desirable aesthetics, I’m concerned that this is a poor choice for accessibility, given that this is not an input, it’s an output. Is a different HTML Element more appropriate here?

Here is a small reproduction of what I’m trying to do:

<h1>Triangle Area Calculator</h1>
<form>
    <ul>
        <li>
            <label for="base">Triangle base:</label>
            <input type="number" step="any" id="base" name="base" required onchange="checkComplete()">
        </li>
        <li>
            <label for="height">Triangle height:</label>
            <input type="number" step="any" id="height" name="height" required onchange="checkComplete()">
        </li>
        <li>
            <label for="area">Triangle area:</label>
            <input type="number" step="any" class="result" id="area" name="result" readonly>
        </li>
    </ul>
</form>

form ul {
  list-style: none;
}
input.result{
  background-color: whitesmoke;
}

function checkComplete() {
    base_input = document.getElementById("base")
  height_input = document.getElementById("height")
  area_result = document.getElementById("area")
  
  if (base_input.checkValidity() && height_input.checkValidity()){
    area_result.value = base_input.value * height_input.value/2
  }
  else{
    area_result.value = ""
  }
}

Here is the above in a JSFiddle project.

>Solution :

<output>: The Output element

"use strict";

function checkComplete() {
  let base_input = document.getElementById("base");
  let height_input = document.getElementById("height");
  let area_result = document.getElementById("area");

  if (base_input.checkValidity() && height_input.checkValidity()) {
    area_result.value = base_input.value * height_input.value / 2;
  } else {
    area_result.value = "";
  }
}

form ul {
  list-style: none;
}

input.result {
  background-color: whitesmoke;
}

<h1>Triangle Area Calculator</h1>
<form>
  <ul>
    <li>
      <label for="base">Triangle base:</label>
      <input type="number" step="any" id="base" name="base" required onchange="checkComplete()">
    </li>
    <li>
      <label for="height">Triangle height:</label>
      <input type="number" step="any" id="height" name="height" required onchange="checkComplete()">
    </li>
    <li>
      <label for="area">Triangle area:</label>
      <output for="base height" class="result" id="area" name="result"></output>
    </li>
  </ul>
</form>