selector-required-ref.html
<!DOCTYPE html>
<html>
<head>
<title>CSS level4 Selector :required and :optional</title>
<link rel="author" title="LEE YUN HEE" href="mailto:zzirasi@gmail.com">
<style>
input {border:3px solid green;}
</style>
</head>
<body>
<p>
You should see a green input box.
</p>
<p>
<input>
</p>
<p>
<input>
</p>
</body>
</html>
