.class

css/sytle.css

.a {
color: #0F0;
font-size: 36px;
}

.a {
 color: #0F0;
 font-size: 36px;
}

index.html

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