There is no install procedure. The files just need to reside in the same folder on a webserver that has PHP installed, and then the two files:
You can use the browser's zooming capabilities to enlarge the clocks once they are running (if screen space permits).
The download of the webpages requires an internet connection, but once the clock page is displayed, the connection can be terminated (or lost) with no consequences.
You can in fact use the setup page and save the values, open the clock page and then use your browser's capability to save the page locally as pure HTML. This page can then be opened locally and run with no internet connection. The only drawback is that you cannot change the values (except by going through the setup and save procedure once again).