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<!DOCTYPE html>
<html>
<head>
<style>
i:first-child {
  background: yellow;
}
</style>
</head>
<body>
<h1>Demo of :firstchild</h1>
<p>I am a <i>strong</i> man. I am a <i>strong</i> man.</p>
<p>I am a <i>strong</i> man. I am a <i>strong</i> man.</p>
</body>
</html>