Using full W3C standard script to get a select element's selection is tricky, and probably involves some messing about to get all browsers to conform. I'll have to look deeper sometime. The warning isn't necessarily about not using fully W3C DOM1 interface code, it's just that you are relying on using the form's name as a global reference.
Either of these 2 statements should make the warning go away:
Hmm.. DTD -- I would like to learn that. Where can I get started for that part?
I already changed my document to conform your a. part suggestion, at least for now. I'm using Firefox, IE (all versions), Opera and lynx (paramount important) to test my application development. Warning went away except for document part.