console 命令行工具 X clear
console
<!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>
  <style>
    body {
      --a: red;
      --b: 100px;
    }

    .bbb {
      background: var(--a);
      width: var(--b);
    }
  </style>
</head>

<body>
  <div>111</div>
  <iframe src="./test.html" frameborder="0"></iframe>
  <script>
    console.log(111, top == self);
  </script>
</body>

</html>