A good place to fiddle with HTML, JS, and CSS in a browser.

Mustache.js   – A JavaScript library (has a Ruby gem as well) to HTML template creating.  I primarily use ERB. Not sure at this point how this would benefit.

SVG HTML element to create graphics in HTML.

Unity game engine a cross-platform game engine.

matter.js A simple 2D physics based animation JavaScript library.

Phaser framework – A light-weight JavaScript library to create simple game. I have to say it is more mature than matter.js…