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Bootstrap
The most popular HTML, CSS, and JavaScript framework for developing responsive, mobile first projects on the web.
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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.
100% Flex box is the way to go. Thereās a great game for learning too: https://flexboxfroggy.com šø
Other people have posted great solutions, though personally I take the easy way out and use bootstrap, which is a css library with very good documentation and pretty much designed for mobile-responsiveness - here's a link to the main page https://getbootstrap.com/