<!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>