CodeSandbox is an online editor that helps you create web applications, from prototype to deployment.
You may also be interested in ...
Protomaps
Protomaps is an open source map of the world, deployable as a single static file on cloud storage.
HTML Reference
HTML Reference.io is a free guide to HTML. It features all elements and attributes.
Compare dates in JS
Learn how to compare two dates in JavaScript with various methods by Saruque Ahamed Mollick.