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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.
But it's command like tool, I don't think that there is Online tool even smilar to the Python tool you showed. You can try creating issue with a question on https://github.com/gambit/gambit/issues Marc Feeley may consider that it's also nice idea to show usage of Gambit, he was working on making Gambit on new try.scheme.org website, I've also did some help.
This is interesting challange maybe I will write one for my Scheme implementation. It would be the only online tool like this for Scheme in Browser, unless Marc Feeley decide to create one for Gambit.