When comparing FLTK vs Sciter, the Slant community recommends Sciter for most people. In the question“What are the best cross-platform GUI toolkits?” Sciter is ranked 14th while FLTK is ranked 16th. The most important reason people chose Sciter is:
Only a single native DLL.
Ranked in these QuestionsQuestion Ranking
Pro GUI designer
Fast Light User-Interface Designer (FLUID) included.
Uses a limited and lightweight design and restricts itself to solely GUI functionality. Because of this restriction, the FLTK hello world example is only about 100 KiB.
It's simple design and lack of more advanced C++ features makes it easy for beginners.
Con Does not look like a native application
Because it uses non-native widgets, it doesn't look like a native application on any platform.
Con Can be too simple for some projects
FLTK offers far fewer widgets than most other toolkits.
Con Linux version is not very mature
The Linux version is missing HTML/CSS features when compared to the Windows version.
Con Not fully HTML5 compliant
Lacking HTML5 functionality and W3C standards: grabbing a library like JQuery or Bootstrap and use it in Sciter will not work.
Con Not WYSIWYG
Not WYSIWYG like WebForms or WPF.