index.html

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="utf-8">
        <meta http-equiv="x-ua-compatible" content="ie=edge">
        <title>Морской бой</title>
        <meta name="description" content="">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <link href="style.css" rel="stylesheet">
    </head>
    <body>

        <div id="gameArea">
            <div class="footer htmlpreview-fallback">
                <button class="btn" onclick="runGame(true)">Начать игру</button>
            </div>
        </div>

        <script src="index.js"></script>
        <script>
        function runGame(startImmediatly) {
             var game = new SeeBattle('gameArea');
             game.run();

             // fallback для htmlpreview
             if (startImmediatly) {
                game.startNewGame();
             }
        }
        </script>
    </body>
</html>