You (mostly) don't need Apple's four-year-old ebook template: Because the export recognizes page breaks and section breaks, you can use those along with custom text styles to create your ebook table of contents — you don't have to download Apple's template. That said, Pages takes a shortcut here with its text styling; if you look at the code of your resulting export, all text styles come out as various CSS classes for the paragraph tag. If you want header tags, however, you'll still have to use Apple's ebook template.
Other fun stuff you can do: You can style text with small caps, use superscript or subscript, add character spacing, and indent your text; lists now render properly with list HTML tags; tables render as selectable tables, rather than images, and can be color-customized; floating images will render as static images, rather than disappearing altogether; and if you export a document that has tracked changes pending, it automatically accepts all changes when rendering.
Things that still need work
Images still disappear if you've added too many to a chapter: Known as the "Over 11MB" rule, Pages automatically removes any images over 11MB per chapter in order to keep your file size manageable. Unlike past versions, it now gives you a specific warning when this happens, but it's still annoying. I would much rather have all images in the book shrunk to lower resolution than have them disappear entirely. You can trick the exporter into "breaking" chapters by using Apple's template and pasting dummy "Chapter title" text every so often, but you'll then have to open the ePub using Sigil or a code editor to remove those blank spaces.
When in doubt, Pages converts elements to images: Despite the fact that the CSS stylings exist for adding borders, if you attempt to apply any to your images or inserted text, Pages flattens them onto the element in the export process, making a new image. It's a clunky way of handling what should be relatively simple CSS code.
CSS tricks that should work, don't: On the Web, you can add shadows and outlines to your text, put borders on your images and captions, stagger your line-spacing, keep images and captions together, make pop-up footnotes and embed custom fonts. You can do the same in a Pages document, but none of it translates when you export your ebook. I'm crossing my fingers that these slowly get addressed in future updates to the export engine — the underlying code for all this exists, it just needs to be incorporated. (Custom fonts, in particular, would be a nice improvement.)
The code is still messy, in part due to CSS handling: There aren't any major preference screens to go through when exporting your ebook, which is good for usability, but can be frustrating for mapping CSS templates accordingly. All of Pages's text styles get exported as various CSS paragraph classes, and they change on every export. The "s1" class may refer to the Pages "Title" style during one export or "Body" during another, depending on which text style comes first in your manuscript. And unless you use Apple's ebook template, you won't have any heading tags at all. It's something that could easily be remedied during the export process with an extra screen that asks you to map your text styles to HTML tags like h1, h2, h3, caption, and such.
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