<!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>