One thing is what the CSS 2.1 and CSS 3 specs say, another thing is what ePub readers actually do (note that ePub 2 is based on CSS 2, not 2.1, so the px definition is a bit more ambiguous). I have seen readers say "10cm" was something like 8.5 cm, and that an A4-sized PDF displayed to fit the 6" screen was at "100%"...
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