In the next couple of posts, I hope to give you an idea of what works well with the ePub export, how to get better ePub documents from InDesign, and what sort of documents would work better as a PDF. To that end, I thought I should start with giving you some background on what an ePub is and how the Digital Editions export inside InDesign works to create ePubs.
You may be aware that ePub is really made up of OPS, OPF and OCF, which are standards from the International Digital Publishing Forum (www.idpf.org). They build on other standards, including XML and XHTML. This means that the pages you're seeing in Digital Editions (when you export an ePub from InDesign) are XHTML. They have xml structures around them to aid navigation and presentation, and a package that wraps it all so that the many files are a single item on disk. Still, they are essentially XHTML and CSS, and that means they have certain strengths and weaknesses.
In building the current version of the Digital Editions export into InDesign, we made the decision to focus on book formats that lend themselves easily to automated layout - that is to say, narrative books where readability is important but the view of a double page spread or of a single page isn't a priority. Still, the XHTML nature and the focus on narrative books means that certain items won't be preserved when exporting ePubs from InDesign.
There's no support for putting text on a path in XHTML. When you put text on a path in InDesign that text would need to either be rendered as a graphic, or it looses the path. For CS3 we decided not to export the text as a graphic, so the ePub export preserves the text, but not the path, so that the text is still text in the XHTML. The same is true for text that is set at an angle. If you create a text frame in InDesign, and then rotate it a few degrees one direction or the other, the text will still be exported as XHTML, rather than choosing some other format. This means that the rotation is not exported, just the text. Of course if you have a document that uses these features, they're preserved perfectly when you export a PDF and open that in Digital Editions.