Ironically, the only reason we have to use invalid CSS is to increase browser support. border-radius and box-shadow are a part of the CSS3 spec by now, but older browsers only support the prefixed versions. After all, the whole point of using normalize.css was to get things to look consistent between browsers.
You make a great point about vendor prefixes, but I can't blame browser vendors for implementing features faster than they are standardized. It's certainly better than being the other way around. I have faith for now that the scenario you point out will never happen.
Also, non-standard code also led to some useful technologies like AJAX. The blink and marquee tags just tell one side of the story. Just pointing that out.
I care about standards as much as anyone and I understand FIRST's intention, but I think it makes sense to stretch the rules a bit.