внешний стиль

css/style.css

p {
color: #F00;
font-size: 50px;
}

p {
 color: #F00;
 font-size: 50px;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
</head>
<body>
<p>Hello World!</p>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
</head>
<body>
<p>Hello World!</p>
</body>
</html>