Obviously niche, but the GOV.UK Design System is very well supported in Rails.
There's a form builder and library of components (built and maintained by me) which, between them, provide full coverage. Most new Rails services use them.
It's a good match because accessibility is one of the Design Systems' primary concerns so JavaScript is used very sparingly and only to enhance.
They're not official, only Nunjucks is supported by the Design System team.
https://design-system.service.gov.uk/
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