On Wed, 20 Nov 2013, Keith J. Schultz wrote:

> 	\usemodule[ebook]
>
> 	\setupcss[…]{…}% see comment #1
>
> 	\setupmapping[…]{…} % used for when author has his/her own ideas #2
>
> 	%normal ConTeXt sets see comment #3
>
> 	% possibly set a mode or set externally
>
> 	\starttext
> 		\startebook
> 			\chapter… %see comment #4
> 				\startparagraph{leftmargin=20%, …] % see comment #5
> 				% text
> 				\stopparagraph
> 				\starttable…
> 				\stoptable
> 				…
>                \stopebook
> 	\stoptext

To me, the biggest advantage of a TeX based system is the ease of 
extensibility. If you want to restrict to a specific subset, then might as 
well use XML:

<document>
<book>
<chapter>
<paragraph leftmargin=20%>
  text
</paragraph>
</book>
</document>

or using one of the existing XML schemas rather than inventing your own 
(perhaps even HTML5).

As far as ConTeXt is concerned, you can process the above XML quite 
easily. Come to think of it, it may be a useful to provide a module that
maps HTML5 to PDF.

Aditya