Not only is the Calibre UI awful, but the HTML it generates internally (e.g, when converting to ePub) is seriously nasty. I wouldn't be surprised if it significantly slowed down book opening / display on some devices.
Part of the problem, though, is simply that there's no really good common "interchange format" for ebooks. All of the currently available formats (ePub, MOBI, HTML, PDF, FB2...) are effectively meant for direct consumption by some sort of device or another, so converting them to other formats is a messy process. I'm currently working on a simple new format for authoring (fiction) books in, as well as tools for converting this to all the other formats that matter; anyone interested in helping? :)
The format I'm planning to use as the intermediate is basically just a restricted subset of HTML. I'm primarily targeting fiction texts with this format, so advanced formatting isn't generally necessary.
Part of the problem, though, is simply that there's no really good common "interchange format" for ebooks. All of the currently available formats (ePub, MOBI, HTML, PDF, FB2...) are effectively meant for direct consumption by some sort of device or another, so converting them to other formats is a messy process. I'm currently working on a simple new format for authoring (fiction) books in, as well as tools for converting this to all the other formats that matter; anyone interested in helping? :)