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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.
If you know CSS, try getting familiar with Sass. It is a more organized way of writing CSS that can be compiled with your site.
Bootstrap is a well known and often used web framework. It has great documentation to learn how the pieces work together - HTML, CSS, and javascript. And responsive design (making your code work on desktop, tablet, and mobile).