Is there a way for the HTML5 page to access the serial port of the device, all on the client side? I know that this can be done using a Java applet, but I would like to know if this can be done using HTML5.
He can not. Browsers do not allow port access without using a plugin.
Google Chrome has a javascript API for this ( http://developer.chrome.com/apps/serial.html ) but it is browser specific, HTML5
Not. And no JavaScript can be. You mix Java (applets) and JavaScript. They have nothing to do with each other.