Usually I rewrite it anyway to make it simpler and because my pages are different from theirs, but strictly speaking I do not honour their copyright because I do not acknowledge it. Conversely, none of the scripts on this site are copyrighted, so use whatever you like in any page you want. If you're really happy with a script you might add the URL of this site in a comment, but even that is not required. Any JavaScript should work in as many browsers as possible. In addition, I don't waste time on scripts that do not work in at least one Netscape version and one Explorer version.
Life's too short to chase all proprietary extensions the various browser vendors have thought up.
Any JavaScript should be as short as possible . I still find it strange that people can write immensely complicated scripts where a few simple lines will serve. This goes especially for the scripts generated by the various wysiwygs. There are only simple scripts on this site. This is deliberate.