Just write in plain text on your computer, and sync with GitHub or Dropbox.  Or use our in-browser text editor.
​
The decision of "is your plain text considered Leanpub Flavoured
Markdown or Markua" is just a checkbox you click at
​https://leanpub.com/YOURBOOK/edit_manuscript_format

The long version of the Markua spec is here:
​https://leanpub.com/markua/read
​
The "what's not supported yet in Leanpub list" is here, and it's
pretty complete and accurate:
​https://leanpub.com/markua/read#leanpub-auto-todo-in-leanpub
​
The (outdated) disclaimer about planned release dates is here:
​https://leanpub.com/markua/read#leanpub-authors

We let the schedule on Markua slip because of adding support for
courses, which is launching in beta next week.  Markua should be 100%
complete near the end of 2019.
​
Right now Markua is basically at feature parity with Leanpub Flavoured
Markdown.  Leanpub Flavoured Markdown is fully documented here:
​
​https://leanpub.com/help/manual
​
So, if you want an experience where everything you see documented is,
in fact, supported, just use Leanpub Flavoured Markdown and see the
reference at https://leanpub.com/help/manual
​
The differences between Leanpub Flavoured Markdown (LFM) and Markua
for the basic stuff you do are pretty minimal.  The biggest differences are listed here:
​
​https://leanpub.com/markua/read#leanpub-auto-differences-with-leanpub-flavoured-markdown
​