You can detect when an HTML5  has finished playing with an 'ended' event listener, and update the CSS there:
<video src="video.ogv" id="myVideo">
  video not supported
</video>
<script type='text/javascript'>
    document.getElementById('myVideo').addEventListener('ended',myHandler,false);
    function myHandler(e) {
        // Video has finished playing!
        // To update the CSS in your question,
        // select the proper elements through their class
        // and change their style.
        document.getElementsByClassName('videobox')[0].style.visibility = 'hidden';
        document.getElementsByClassName('pagecontent')[0].style.visibility = 'visible';
    }
</script>
If you encounter any problem or question while implementing my solution, please let me know in a comment.