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I wrote a piece for Mozilla Hacks, explaining how CSS length works.
Unfortunately the post was not proofread until a few hours after the publication; thanks Robert and Chris to proofread the post for me. I should work on my English more :'(
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.