<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
body {
    background-color: lightgreen;
}

@media only screen and (max-width: 500px) {
    body {
        background-color: lightblue;
    }
}
</style>
</head>
<body>

<p>Resize the browser window. When the width of this document is less than 500 pixels, the background-color is "lightblue", otherwise it is "lightgreen".</p>

</body>
</html>

Sed ut perspiciatis...

Resize the browser window. When the width of this document is less than 500 pixels, the background-color is "lightblue", otherwise it is "lightgreen".