[gtk40]

Nope, it uses https://www.fltk.org/

[axus]

FLTK was a pleasure to use (for uncomplicated software). They also put the latest code on Github: https://github.com/fltk/fltk

[1313ed01]

I just started it up and it turned out to be Dillo 3.0 from 2011. I do not know if it was using FLTK back then, but a quick search says that FLTK has been ported to DOS so that might not be an obstacle for the current developers to keep FreeDOS support if they wanted to.