stefcab is correct. It's a bit more standardized that that implies, but it's a good summary.
On a separate topic (not relating to [Login to see the link]'s reply) ...
There is no reason an ePub would be identical to a "True PDF" in regard to markup. The definition of "True ePub" (not actually a true definition, but the definition by those who use it) -- is that the ePub is generated natively, rather than converted. Therefore, by definition, ePub and True PDF generally have no consistency and this is intended.
I believe that people often forget the entire purpose of eBook formats. Whether it's ePub, Mobi, or Azw, they all have one fundamental purpose above all others: To be device/page agnostic.
Simply put: a PDF is created by rendering the content pixel-perfect as it would be sent to a printer (if you go back to the PS days and the first gen of PDF's, you'll see this is how they were invented; it's basically a portable postscript capture).
eBook Formats, are intended to do the opposite. Contrary to preserving layout, an eBook by definition must allow the layout to be "re-paginated", or in simpler terms, does not require a consistent page size, font size, or any form of layout that would inhibit the eBook from dynamically adapting.
If you are attempting to "compare" eBook formats to PDF, you're missing the point entirely.
To avoid any further misinformation, I'll provide more explanation about the ePub standard in a way that can also help the original poster...
ePub Standard Overview
1) ePub is a .ZIP file, this is not optional, this is explicitly defined in the formal specification.
2) If you open the ePub file as an archive, you will see a file-structure. This file structure is specified by the ePub standard; it should be consistent with all ePub files:
3) If you wanted to see an eBook without formatting, allowing your eBook reader to use its default formatting, you could simply delete the stylesheets in the root directory (page_styles.css and stylesheet.css).
4) No eBooks should have inline styles. It's possible they could have inline styles; however, it's extremely unlikely because 99.9% of eBooks are created using an automated tool (not by hand). Therefore -- deleting the stylesheet should accomplish the goal of removing any formatting included with the book itself.
eBook tools do one of two things, either (a) to create them from the original text, or (b) to covert them from another format. Those tools, abiding by the standard, should combine all styles used into the CSS pages in the root directory.
I believe this is a good start and it also helps the original poster diagnose any problems.
If you still have more questions or would like more technical details on the ePub format, I am happy to answer them.