DOS ain't dead

If someone is interested in updating the things, the best bet is old Google Code page site (while it still exists). Don't get overwhelmed with the number of different files.

I'm pretty sure you only need two things:

The very first file uploaded by Georg was nanox-dos-src-170911.zip from 2011: https://storage.googleapis.com/google-code-archive-downloads/v2/code.google.com/nanox-microwindows-nxlib-fltk-for-dos/nanox-dos-src-170911.zip. That contains the inital port of Nano-X + Fltk 1.3. There is also a document that documents how Georg ported the stuff to DOS at .\microwin\Nano-X-2.pdf. The root of the archive also contains a Readme file that should get you started.

There is also a copy of his DJGPP folder from 2012 that was used to compile stuff: https://storage.googleapis.com/google-code-archive-downloads/v2/code.google.com/nanox-microwindows-nxlib-fltk-for-dos/djgppn.zip. This should also include all the other things including nano-x, fltk, wattcp and openssl (which is out of date, so it should be updated).

Georg didn't post the source for Dillo. If you are lucky that might mean that the build is straightforward and you only need to run configure script to generate a makefile. That can be very slow on DOSBox or on machines weaker than Pentium. DOSEmu (Linux only!) will work much better. Check out DOSEMU2: http://dosemu2.github.io/dosemu2/.

Makeing a more modern toolkit or a cross-compiler and updating or porting the newer versions of libs might be much more involved. But, if you are lucky, it might be small enough for a weekend project. If you are unlucky you are going to end up debugging what and why it doesn't work for several days until you (hopefully) get it to work.