Hi!
thanx Alexander and Pilotbob for your suggestions!
I'll see into firebug as soon as I found out if that's available for MacOS 9x...
For now I found a "Javascript verifier" called "JSLint" by douglas crockford; that's (just so) enough for my humble needs though not very comfortable. I found that at
http://www.jslint.com/lint.html.
Now I hope there's available a firefox for older macs. (I CAN run OS 10.4 but rather will notwith my old maschine...)