You say that light markup (and you use markdown as an example) can handle anything by
including xhtml which means a viewer app that is designed to view a lightweight markup language will have to parse xhtml anyway to display the file. In which case any viewer app advantage in using light weight markup is negated. Incidentally I actually use markdown and have even contributed patches to the python markdown project, so try not to jump straight to the "you dont know what you're talking about" defense. It leaves me with the feeling that you dont have any real points to make.
As for authors not wanting to learn markup. Those that are too lazy to learn markup will be too lazy to learn lightweight markup as well. They will demand a WYSWYG GUI to take care of the markup for them.
You have the attitude that creating a markup language that is just sufficient for all of todays needs is the right approach. You'll then "add more features" as you see the need. But it's not easy to "add features" to a lightweight markup language. Case in point is markdown and how you have to jump to html for any advanced features.
So yes, it is more effort to develop applications for authoring/converting/viewing a "heavy" markup language, but in the end its worth it. To say that we must limit ourselves to a lightweight language simply because developing applications for a heavy language is too difficult, is ridiculous. Let me leave you with the example of TeX. A publishing system that is not lightweight and that has lasted decades.
Lightweight markup is a good fit for gutenberg, but little else. And even there, I suspect they'd have a hard time getting their digitizers to follow the rules. As far as creating modern digital books, there is really no reason to be restricted to a lightweight markup language. And note that I continue to call it lightweight, because that is precisely what it is.
You say you want to maintain a mirror of gutenberg. An excellent idea. If you support export of gutenberg texts to HTML, I might even use it
