It won't be used in any HTML page - it's for development and testing purposes only. If you embed the applet tag in the Java source code like his, then you can use appletviewer to run the applet by typing "appletviewer applet4.java". That's because appletviewer doesn't care about the rest of the file - it only looks for the applet tag.
If you want to deploy the applet for others to use, then you do need a proper HTML page (which would contain the same applet tag).
Ping & DNS - updated with new look and Ping home screen widget