Ouch!

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    body {
      display: flex;
      justify-content: center;
      align-items: center;
      height: 100vh;
    }

    .box {
      width: 300px;
      height: 200px;
      border: 1px solid black;
      position: relative;
      z-index: 1;
    }

    .box::before {
      content: '';
      display: block;
      width: 100px;
      height: 100px;
      position: absolute;
      left: 50%;
      right: 50%;
      top: -51px;
      transform: translateX(-50%) rotate(45deg);
      border: 1px solid black;
      background-color: cornflowerblue;
      z-index: 2;
    }
  </style>
</head>
<body>
  <div class="box">

  </div>
</body>
</html>