Just open index.html in a browser.

Edit /etc/xdg/lxsession/LXDE/autostart to turn off screensaver

#@xscreensaver -no-splash
@xset s off
@xset -dpms
@xset s noblank

Add a bash script in /home/pi/Desktop/clock to launch it in chromium kiosk mode:

#!/usr/bin/env bash
chromium-browser --kiosk --app=file:///home/pi/stageclock/index.html

Don't forget to chmod +x /home/pi/Desktop/clock

Edit /home/pi/.config/lxsession/LXDE-pi/autostart to load it automatically on boot:

@/home/pi/Desktop/clock