<p class="controls">
        <input type="button" value="Play" id="video-btn">
        <span id="curtime">0</span>/<span id="duration">0</span>
    </p>
</p>
<script>
    window.onload = function(){

        //get references to the elements
        var player = document.getElementById("player");
        var btn = document.getElementById("video-btn");
        var curtime = document.getElementById("curtime");
        var duration = document.getElementById("duration");
        ***duration.innerHTML = player.duration;***

Sila beritahu saya, mengapa anda perlu menetapkan duration.innerHTML = player.duration di sini, adakah ia masih berfungsi jika anda tidak menetapkannya?