<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Calculator</title>

  <style>
    body {
      max-width: 740px;
      line-height: 1.3;
    }

    output > p {
      background-color: gray;
      padding: 10px;
      word-break: break-all;
    }

    output > p.success {
      background-color: green;
      color: white;
    }

    output > p.error {
      background-color: red;
      color: white;
    }
  </style>
</hea