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<!DOCTYPE html>
<html>
<head>
<style> 
p:nth-of-type(3n-1) {
  background: red;
}
</style>
</head>
<body>
<h1>Demo of nth-of-type()</h1>
<p>The first paragraph.</p>
<p>The second paragraph.</p>
<p>The third paragraph.</p>
<p>The fourth paragraph.</p>
<p>The fifth paragraph.</p>
<p>The sixth paragraph.</p>
<p>The seventh paragraph.</p>
<p>The eight paragraph.</p>
<p>The ninth paragraph.</p>
</body>
</html>