Formspec designer (FD) allows modders to design and preview formspecs right from within the game, for a faster, more streamlined experience. Simply enter the formspec string in the provided text-area, and click "Preview"! FD even supports loading, saving, and deleting formspec scripts.
magnetar47/formspec_designer
In-game formspec prototyping mod for Minetest, with support for loading, saving and deleting scripts.
LuaMIT