[ProDigit]

I personally dislike classes and xml.
They add a lot of garbage to a file.
Like mentioned before, if created in html0, just make sure all coding is done correctly, before adding text to the book, meaning, have the book in whatever format, and have a separate book with the layout and formatting.
Then it's as simple as merging the files.

Notepad++ for Windows, has a very easy way, through color coding, to indicate where an error may be, on a variety of code types, including html.
And through its advanced find and replace options, making chapters is a piece of cake.

I used it to get the bible framework done, with the ~5500 links. It took me only 48 hours to make, and that included learning to use notepad++.

No matter how you look at it,
There was absolutely no way to generate a file of that magnitude with class or xml, and have the ebook reader not completely go haywire.
There also was absolutely no way, to create that document any smaller in size.
Every space, code, formatting has been carefully measured,
Even with a class, I doubt the file could have been created any smaller, although once different font types come into play, it might be.
In HTML0 I refrained myself to just H1-H4 for titles (as I believe on an ebook reader h4, h5 and h6 didn't look like titles anymore compared to the book's text size).

From a coding perspective, html is the easiest to understand, has the least chance on errors, loads the fastest, and is the smallest in size, when done correctly.