I agree with what Caroline Jarrett posted above — this is a very bad idea that doesn't help anyone. I haven't printed off a web page onto paper in years, but what I have done is 'print' pages as PDF documents for reference. Your CSS hack would remove that option too. Look, I get the desire to minimise your environmental footprint, I'm trying to do the same here as much as possible. But there's no need to be snarky about it.