HTML
CSS
JavaScript
HTML
<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
<p>蓝色</p>
<div>绿色</div>
<div id="alert">红色</div>
</body>
</html>
CSS
  :root { --color: blue; }
  div { --color: green; }
  #alert { --color: red; }
  * { color: var(--color); }
JavaScript