html5 canvas 多个填充渐变形状

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html ="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>填充渐变形状</title><script src='/images/loading.gif' data-original="js/modernizr.js"></script></head><body><script type="text/javascript">window.addEventListener('load',eventWindowLoaded,false);function eventWindowLoaded(){  canvasApp();}function canvasSupport(){  return Modernizr.canvas;}function canvasApp(){  if(!canvasSupport()){    return;  }else{    var theCanvas = document.getElementById('canvas')    var context = theCanvas.getContext("2d")  }  drawScreen();  function drawScreen(){    //水平渐变值必须保持为0    var gr = context.createLinearGradient(0,0,100,0);    //添加颜色端点    gr.addColorStop(0,'rgb(255,0,0)');    gr.addColorStop(.5,'rgb(0,255,0)');      gr.addColorStop(1,'rgb(0,0,255)');        //应用fillStyle生成渐变    context.fillStyle = gr;    context.fillRect(0,0,100,100);    context.fillRect(0,100,50,100);    context.fillRect(0,200,200,100);            }}</script><canvas id="canvas" width="500" height="500">你的浏览器无法使用canvas小白童鞋；你的支持是我最大的快乐！！</canvas></body></html>