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<!DOCTYPE html>
<html>
<head>
<style>
img {
  position: absolute;
  left: 0px;
  top: 0px;
  z-index: -1;
}
</style>
</head>
<body>
<h1>This is a heading</h1>
<img src="img_tree.png">
<p>Because the image has a z-index of -1, it will be placed behind the text.</p>
</body>
</html>