RGBA

css/style.css

div {
width: 300px;
height: 300px;
background-color: rgba(255, 165, 0, 0.3);
}

div {
 width: 300px;
 height: 300px;
 background-color: rgba(255, 165, 0, 0.3);
}

index.html

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

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