My goal is generally "Works in Firefox, IE, and Safari", but last time I checked it validated fine. In general, I tend to think "it works" is more important than "it's right", but if I can make it right too without totally convoluting or breaking things, I might as well.

I never cut & paste from other sites. I like to understand what I'm doing - it's the best way to ensure that it stays working.

One thing that surprised me - after I got things working in Firefox, it was only about a half day's worth of effort to get them working in IE as well. I was expecting it to be much worse, given all the griping about cross-browser compatibility. So perhaps knowing what you're doing does have some benefits...