here's a screenshot showing the first page of jim's catalog as displayed in "kindle for mac" versus that page on an ipad, two devices which obviously use different rendering engines:
Yes different rendering engines, but consider what else is also different: MOBI compiled by Kindlegen, or rather one of its predecessors. EPUB compiled by Calibre, if I remember right. Kindlegen and Calibre each have their own default CSS. Each rendering engine also has its own default CSS. Each of those devices also has its own unique set of fonts, and unique algorithms on how to map font-related requests onto a particular font actually available on that device. Each device in turn has its own set of user controls that override, or not, that CSS.... So, the idea that an e-book "ought" to look the same on two different devices is incorrect in the first place. The trick is in authoring "correctly" such that an e-book presents "correctly" while at the same time looking considerably differently on each different reader device -- including on two "identical" Kindles say owned by two different customers who have two entirely different reading preferences. When one spends "good money" to buy a commercial e-book where in turn the publisher has paid good money to get some e-book transcriber to "make it look just like the print edition" one realizes just how stupid and counterproductive such efforts are. An e-book SHOULD NOT look just like a clone of the paper edition, doing so JUST DOESN'T WORK! E-books are their own unique media, just as paper and television are their own unique media. Not to imply that two paper editions of the same work "must" look the same, either!