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.