<!DOCTYPE html>
<html>
<head>
<style>
input:required {
background-color: yellow;
}
</style>
</head>
<body>
<h3>A demonstration of the :required selector.</h3>
<p>An optional input element:<br><input></p>
<p>A required input element:<br><input required></p>
<p>The :required selector selects form elements with a "required" attribute.</p>
<p>The :required selector is not supported in Internet Explorer 9 or earlier versions.</p>
</body>
</html>