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.