EPub is probably the most stable of the ebook formats. Mobi is the second-runner; Kindles are popular enough that mobi will be a useful format for a nice long time. Epub allows more complexity, though.
Mobi is proprietary; while the conversion software is free, the format itself is owned (by Amazon), and is also limited--mobi was designed to work on PDA devices when they had very small capacities, and it doesn't allow much in the way of formatting. (Not that most people care. But if there are fancy features--line spacing, tables, indented sections, font changes--epub can support them; mobi can't.)
Epub is a wrapper around a zipped html file. You can take a non-DRM'd epub, change the extension from .epub to .zip, unzip it, and look at how the epub is made. There's a small file that says "THIS IS AN EPUB" and a basic folder structure.
Epubs created by Calibre are more complex than the ones downloaded from AO3. Calibre has a lot more options, and those show up in the epubs even if they're all left with default settings.
Not all ebook readers can deal with all the formatting options of epubs, which allow .css files to be included in the zip set. AFAIK, none of the current mobile ebook devices support all the options; the hardware hasn't caught up with the software capabilities. But the epub file will be able to hold those settings, and eventually we should have devices that support all of them.
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Mobi is proprietary; while the conversion software is free, the format itself is owned (by Amazon), and is also limited--mobi was designed to work on PDA devices when they had very small capacities, and it doesn't allow much in the way of formatting. (Not that most people care. But if there are fancy features--line spacing, tables, indented sections, font changes--epub can support them; mobi can't.)
Epub is a wrapper around a zipped html file. You can take a non-DRM'd epub, change the extension from .epub to .zip, unzip it, and look at how the epub is made. There's a small file that says "THIS IS AN EPUB" and a basic folder structure.
Epubs created by Calibre are more complex than the ones downloaded from AO3. Calibre has a lot more options, and those show up in the epubs even if they're all left with default settings.
Not all ebook readers can deal with all the formatting options of epubs, which allow .css files to be included in the zip set. AFAIK, none of the current mobile ebook devices support all the options; the hardware hasn't caught up with the software capabilities. But the epub file will be able to hold those settings, and eventually we should have devices that support all of them.