Originally Posted by pdurrant
I wouldn't recommend using pixels. That's because the margin the user sees now depends on the resolution of the display. Set a 20px margin, and on a 72dpi screen display the margin will be just over 1/4 inch. On a 166dpi eInk screen they'll be just under 1/8 inch. On a 200dpi eInk screen they'll be 1/10 inch. And on some future 600dpi display they'll be just 1/30 inch.
It's better to use a percentage, but my preference is to use an absolute size unit, like points (pt - 1/72 inch), picas (pc - 1/12 inch), mm, cm or inches (in). All these are supported in the ePub flavour of XHTML/CSS.
good point (ar ar ar). i hadn't thought about that, although i should have because in daffy's great experiment thread i explained how pixel size varies depending on resolution in the same post as i was talking about margins.
i tend to use a percentage, personally ; it seems logical to use something which is proportional to the size of the display. it seems weird to me to use any kind of "physical" measurements (pts, mm, etc.) for digital media even if it can work, but that's probably just my own little neurosis.