ModalBox.html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <link rel="stylesheet" href="../CSS_FILE/ModalBox.css" />
</head>

<body>
    <div id="cont" onload="overlay()"></div>
</body>

</html>

<script src="ModalBox.js"></script>