1. the background should be almost-opaque, with very little transparency. Can you share which browser/OS? Probably some missing CSS :)
2. Ha :D Maybe a grace period? Did you mostly mind the squiggly underline errors, or the red box that appears at the bottom? Giving feedback soon enough but not being aggressive is a delicate balance, there's probably room for improvement here.
2. Maybe that could work. A red underline is fine but the error at the bottom is the biggest thing. Instead of switching between neutral/error state, why don't you experiment with working/neutral state? If the code is fine, show a green tick at the bottom, else just show the error with grey color and a grey icon so it's visible but not attention grabbing. On the other hand, you'd get the advantage of getting people hooked with getting the code back to green. Positive reinforcement.
2.