Tandem is a visual editor for web applications.
Tandem allows users to write code, and immediately see visual elements appear in an editable interface. Users also have the ability to use Sketch-like UI tools for editing visual elements that produces code in any language that supports source maps (HTML, CSS, SASS, SCSS, React). All of this is powered by Tandem.
The app takes inspiration from many ideas, more notabily Bret Victor's presentation on the future of programming. Tandem is an attempt to provide the APIs necessary for creating realtime programming environments (like Sketch + coding).