Dillo started as a fork of the Armadillo browser which was based on GTK+. It was the next version - Dillo 2 - which was ported to FLTK. The GTK+ version did not support CSS and initially did not support frames/iframes either. I submitted a patch which added support for the latter somewhere around 2003 which was never merged before the port to FLTK but which I have used for quite a while until these features became less important due to the uptake of CSS.
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.