PHP calculator

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document
    </title>
    <title>Simple Calculator Program in PHP - Tutorials Class
    </title>
  </head>
  <div class="bg">
  </div>
  <div id="calc-cart" class="bg bg2">
  </div>
  <div id= "layer 3"class="bg bg3">
  </div>
  <style>
    h2 {
      color: #970000;
      font-family:Arial!important;
    }
    b {
      font-family: arial;
    }
    #calc-cart {
      background-color: bisque;
      display: grid;
    }
    .inputs {
      box-shadow:inset 0px -6px 20px 1px #a4e271;
      background:linear-gradient(to bottom, #89c403 5%, #77a809 100%);
      background-color:#89c403;
      border-radius:33px;
      display:inline-block;
      cursor:pointer;
      color:#ffffff;
      font-family:Arial!important;
      font-size:21px;
      font-weight:bold;
      font-style:italic;
      padding:11px 24px;
      text-decoration:none;
      text-shadow:0px 3px 0px #528009;
    }
    .inputs:hover {
      background:linear-gradient(to bottom, #77a809 5%, #89c403 100%);
      background-color:#77a809;
    }
    .inputs:active {
      position:relative;
      top:1px;
    }
    html {
      height:100%;
    }
    body {
      margin:0;
    }
    .bg {
      animation: slide 3s cubic-bezier(.99,.05,.12,1.02) infinite alternate;
      background-image: linear-gradient(-45deg, #cca633 50%, #ac5845 50%);
      bottom:0;
      left:-50%;
      opacity:.5;
      position:fixed;
      right:-50%;
      top:0;
      z-index:-1;
    }
    .bg2 {
      animation-direction:alternate-reverse;
      animation-duration:4s;
    }
    .bg3 {
      animation-duration:5s;
    }
    .content {
      background-color:rgba(255,255,255,.8);
      border-radius: 18%;
      box-shadow: rgba(255, 255, 255, 0.1) 0px 1px 1px 0px inset, rgba(50, 50, 93, 0.25) 0px 50px 100px -20px, rgba(0, 0, 0, 0.3) 0px 30px 60px -30px;
      box-sizing:border-box;
      left:50%;
      padding:10vmin;
      position:fixed;
      text-align:center;
      top:50%;
      transform:translate(-50%, -50%);
    }
    h3 {
      font-family:Arial, Helvetica, sans-serif;
    }
    @keyframes slide {
      0% {
        transform:translateX(-25%);
      }
      100% {
        transform:translateX(25%);
      }
    }


/* Inline #1 | https://localhost//phpstuff/calci2.php */


</style>
<?php
$first_number = $_POST['first_number'];
$second_number = $_POST['second_number'];
$operator = $_POST['operator'];
$result = '';
if (is_numeric($first_number) && is_numeric($second_number)) {
    switch ($operator) {
        case "Add":
           $result = $first_number + $second_number;
            break;
        case "Subtract":
           $result = $first_number - $second_number;
            break;
        case "Multiply":
            $result = $first_number * $second_number;
            break;
        case "Divide":
            $result = $first_number / $second_number;
    }
}

?>

<body>
<div class="content">

    <h2 style = "font-size:30px;">PHP - Simple Calculator Program</h2>
      <form action="" method="post" id="calc-form">
            <p>
                <input type="number" name="first_number" id="first_number" required="required" value="<?php echo $first_number; ?>" /> <b>First Number</b>
            </p>
            <p>
                <input type="number" name="second_number" id="second_number" required="required" value="<?php echo $second_number; ?>" /> <b>Second Number</b>
            </p>
            <p>
                <input readonly="readonly" name="result" value="<?php echo $result; ?>"> <b>Result</b>
            </p>
            <input type="submit" name="operator" value="Add" class="inputs" />
            <input type="submit" name="operator" value="Subtract" class="inputs"/>
            <input type="submit" name="operator" value="Multiply" class="inputs"/>
            <input type="submit" name="operator" value="Divide" class="inputs"/>
      </form>
    </div>

</body>
</html>