HTML5 Canvas

HTML5 Canvas is a way to draw arbitrary graphics on a web page. You use it by writing code in JavaScript.

While JavaScript is not a particularly speedy language, modern computers are so fast that the JS/browser platform is about as capable as a PC from the mid-1980s or so. Anything that ran back then should be implementable in JavaScript today. This opens up a rather large vista of retrocomputing possibilities.

Canvas Tutorial
Canvas Reference
ACME Labs / Canvas