How accurate are CSS's real-world units?

I'm beginning experiments with CSS' real-world units, specifically, in and cm, for purposes of printing.

Are these units based on how they w开发者_如何转开发ill be displayed on-screen or how they will be printed by a printer? How accurate are these units?

According to W3C, only em, px and % are recommended for screen, the others for print.

Therefore, in a print application, the in and cm measurements could be precisely accurate.

There is an excellent article on such applications here:
http://meyerweb.com/eric/articles/webrev/200001.html