This is a list of documents to start with:
Currently, a document Problems with FLTK is maintained, ideally, it will be removed soon.
There are several classes etc., which are used for tasks formerly (in the GTK+ version of dillo) achieved by GtkObject (in 1.2.x, this is part of Gtk+) and GLib. For an overview on all this, take a look at Lots of Useful Tools.
GtkObject is replaced by the following:
Hash tables, linked lists etc. can be found in the container namespace, several useful macros from GLib have been implemented as inline functions in the misc namespace.
As an alternative to the macros defined in list.h, there is also a template class, misc::SimpleVector, which does the same.
If you have been familiar with Dw before, take a look at Changes to the GTK+-based Release Version.
1.5.9