Which selector places the elements it styles behind all other elements?

<html>
<head>
<style>
img { position: absolute; z-index: -1; }
h1 { color: #ffffff; }
h2 { z-index: 1; }
</style>
</head>
<body>
<img src="https://mimo.app/i/cat.png">
<h1>Cat</h1>
<h2>Whiskers</h2>
</body>
</html>]]>imgh1h2

Make sure to pick img.