EPUB and MOBI books open showing the first chapter and lay out the rest as you read, so very large books open in a fraction of a second. Your reading position is saved as a chapter bookmark, so it survives font-size changes. With ShowChaptersInEbooks = true, the toolbar shows Chapter: n / N alongside the page within the chapter, and Go to Page asks for chapter and page.

EPUB and HTML documents now show CSS background images, so fixed-layout EPUBs made by scan-to-EPUB tools no longer render as blank pages (#6131).