Refactored

<!doctype html>
<html>
    <head lang="en">
            <meta charset="utf=8">
            <title>Exercise</title>
    </head>
    <body>
        <h1>Exercise</h1>
        <h3 id="message"></h3>

        <script>
            var result = 1;
            var numInput = Number(prompt("Factorial calculation: please enter a number"));

            for (i = 2; i <= numInput; i++) {
            result = result * i;
            }

            var htmlRef = document.getElementById("message");
            htmlRef.textContent = numInput + "! = " + result;
        </script>
    </body>
</html>