<!doctype html>
<html>
	<head>
		<title>ID</title>
		<link href="style.css" rel="stylesheet">
	</head>
	<body>
		<h1 id="title">This heading will be red</h1>
		<p>This paragraph will be black, with no ID.</p>
		<p id="introduction">This paragraph will be blue.</p>
	</body>
</html>
index.html
			#title {
	color: red;
}

#introduction {
	color: blue;
}
style.css