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headlessui
Completely unstyled, fully accessible UI components, designed to integrate beautifully with Tailwind CSS.
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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.
IMO, there isn't a lot about Svelte that makes it easier/harder to make an accessible app than the other big frameworks. It does have built in compiler warnings when you write inaccessible markup, but those only cover a small subset of a11y issues. Other frameworks have similar tools as linting plugins, e.g. React has eslint-plugin-jsx-a11y.
I'd love to see svelte support added to to projects like headlessui but understandably as a new library they want to limit the scope and push forward to a stable release before supporting smaller frameworks like svelte. They have react and vue components for now.