ObservableHQ notebooks for JavaScript demos and prototypes

ObservableHQ is a platform being built by Mike Bostock (creator of the D3 visualisation library), Jeremy Ashkenas ("Made CoffeeScript, Backbone.js, Underscore and other ragbag" from his Twitter bio) and Tom MacWright (creator of the big presentation framework, simple-statistics and documentation.js as well as D3 contributor amongst other things).