index.html (example 1)
<!doctype html>
<html lang="en">
<head>
<title>BuckysRoom</title>
<link rel="stylesheet" href="index.css">
</head>
<body>
Hello
</body>
</html>


index.css (example 1)
@media screen and (max-width: 800px) {
body{
background-color: blue;
font-size: 40px;
}
}


index.html (example 2)
<!doctype html>
<html lang="en">
<head>
<title>BuckysRoom</title>
<link rel="stylesheet" media="screen and (max-width: 800px)" href="index.css">
</head>
<body>
Hello
</body>
</html>


index.css (example 2)
body{
background-color: blue;
font-size: 40px;
}