gecko/layout/reftests/image-element/canvas-drawing.js

function createCanvas() {
  var canvas = document.createElement("canvas");
  canvas.setAttribute("width", 100);
  canvas.setAttribute("height", 100);
  var ctx = canvas.getContext("2d");
  ctx.fillStyle = "rgb(200,0,0)";
  ctx.fillRect(10, 10, 55, 50);
  ctx.fillStyle = "rgba(0,0,200,0.5)";
  ctx.fillRect(30, 30, 55, 50);
  return canvas;
}