The "body" element does not add margins to individual pages.
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By default, Atlantis Word Processor saves no page margins to EPUBs. In this way, eReaders display the Atlantis EPUBs with their default page margins. If you enabled the "Save page margins" option in the "File | Save Special | Save as eBook..." dialog of Atlantis, you can click the "Setup..." button to specify page margins for your eBooks. Note that ADE does not support page margins specified as percentage values. Please click the "Setup..." button in the "Save as eBook" window of Atlantis, and switch to another measurement unit. By default, Atlantis offers "cm".