var canvas = document.getElementById('canvas');
if (canvas.getContext) {
  var ctx = canvas.getContext('2d');
  
  ctx.fillRect(25, 25, 100, 100);
  ctx.clearRect(45, 45, 60, 60);
  ctx.strokeRect(50, 50, 50, 50);
  
  // 红色矩形
  ctx.beginPath();
  ctx.lineWidth="6";
  ctx.strokeStyle="red";
  ctx.rect(0,0,150,150);
  ctx.stroke();
}