this is some nice code, which should be quite portable. However it does have some unhelpful omissions:
1) It should be mentioned that the form needs to be called like <form action="" onSubmit="return functionName();" etc..> omitting this assumes a detailed knowlege of JS inappropriate for the level of the document.
2) Some of the functions detailed in the web pages (not the js source doc) do not include:
at the end. This again is crucial to the script working correctly and reduces the value of this otherwise good tutorial.