Observable is a new way to write interactive simulations in the browser. It looks like a pragmatic, powerful and simple way to implement many of the ideas of Bret Victor today in a modern web app.
Created by Mike Bostock
There are some good questions, comments and links over at Hacker News -
ycombinator 
It's been in development for over a year, and was released end of January 2018.
The code is nearly entirely client side, and is planned to be embeddable on any website using a standard javascript library.
# Demos
YOUTUBE uEmDwflQ3xE Observable: An Earthquake Globe in Ten Minutes
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Wrote my first @observablehq notebook today after reading docs for 30 minutes. I explain how federated wiki renders pages with dynamically loaded plugins. If you like coding in the chrome inspector then you are going to love Observable. tweet