As of version 0.4.96 calibre
has a new Ebook viewer with full support for HTML+CSS. The viewer is capable of displaying all the major ebooks formats: EPUB, LIT, MOBI, FB2, PRC, etc It has support for Table of Contents, Meta information, search, fully customizable fonts, etc. An innovative new feature is the "reference mode" that can be used to generate reference numbers for each paragraph in the the text. These allow for easy and accurate cross referencing of ebooks.
It can be launched from the GUI by clicking the View button or from the command line by the command
Note that the old viewer is still used for LRF files. Also note that the viewer doesn't jump through hoops to emulate "pages". It does however have a progress indicator that tells you both approximately how large the book is and how much of it you have finished reading.