This content is over 6 years old. It may be obsolete and may not reflect the current opinion of the author.
I wrote a piece for Mozilla Hacks, explaining how CSS length works.
A negative reference (which was meant to be joke) on inch was also removed after being pointed out by Brendan. I tried to keep the post light-toned but I guess it’s a bit too far.
Anyhow, I hope you find the post on CSS length informational.