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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.
The inimitable Caleb D. Williams has kindly published an ElementInternals polyfill which weighs in at ~6kb over-the-wire. Since the spec involves hooking into browser stuff which is otherwise unavailable to developers, the polyfill is not 100% spec compliant. For example, ARIA reflection is implemented by adding aria- attributes to the host element, where the spec states that they should not be added. The polyfill also adds a workaround for the custom state part of the spec, which was not covered here.