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lazysizes
High performance and SEO friendly lazy loader for images (responsive and normal), iframes and more, that detects any visibility changes triggered through user interaction, CSS or JavaScript without configuration.
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select2
Select2 is a jQuery based replacement for select boxes. It supports searching, remote data sets, and infinite scrolling of results.
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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.
And, lazyloading is only for images. If you use e.g. lazysizes you can do anything with it. I'm a digital publisher so I use it for ads and embedded tweets/IGs and so on.
I've been building this exact thing and am using Select2 which works well but still needs jquery. It really should be standard functionality.
It's nice for adding a basic accordion quickly, but it does not support animation yet. However for projects with animation, you usually have a framework, which makes it easy for you anyway. E.g. Angular Bootstrap widgets.