[scott_s]

I'm less likely to continue this conversation over email. Such is life. :)

I think that bringing in CSS is a bridge too far for the case of writing a book. Authors are no longer just writing HTML, they're writing HTML and CSS. And CSS is intimately tied to actual web browsers - you're now going to need a browser display engine to be an integral part of how you render your book. (Or you're going to need to implement just enough of that yourself, which is a big undertaking.)

The point of comparison is Markdown, which is relatively straight-forward and simple. HTML and CSS is not that.