Try it Live - CSS Pseudo class :optional

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<!DOCTYPE html>
<html lang="en">
​
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>CSS Pseudo class :optional</title>
    <style>
        input {
            border: 1px solid black;
            text-align: center;
        }
​
        input:optional {
            background-color: #F7DED7;
            border: 2px solid red;
        }
    </style>
</head>
​
<body>
​
    <form>
        User: <input type="text" name="user" required>
        <br>
        <br>
        Password: <input type="password" name="pass" required>
        <br>
        <br>
        Email: <input type="email" name="email">
    </form>
​
</body>
​
</html>
​