jsFiddle is the perfect solution; it provides a custom environment (based on user selections) to test (or fiddle with) your JavaScript, HTML, and CSS code right inside your browser.

jQuery UI is another option for building HTML and JavaScript-based application interfaces. It's completely open source and has the advantage of being directly compatible with jQuery, jQuery Mobile, ...

Streamlit lets you write web-based Python data applications without HTML, CSS, or JavaScript. Here's a first look at Streamlit. A common problem with Python applications is how to share them with ...