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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
I decided to make my own userscript GUI library: UserGui. It should ease the process of creating the GUI and let you focus more on your userscript. The project is still in early stages of development, but I am still looking for feedback, such as bugs, annoying features or just the general experience you had setting up the GUI.
The closest I got to using GUI for my user scripts was GM_webextPref, but it was too large in code, and very complicated to fork it and make simpler.