​x
 
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Validation Example</title>
</head>
<body>
    <form onsubmit="return validateForm()">
        <label for="email">Email:</label>
        <input type="email" id="email" name="email">
        <button type="submit">Submit</button>
    </form>
​
    <script>
        function validateForm() {
            var email = document.getElementById("email").value;
            if (email === "") {
                window.alert("Please enter your email.");
                return false; // Prevent form submission
            }
            return true; // Allow form submission
        }
    </script>
</body>
</html>