html5 canvas 奇怪的形状垂直渐变

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="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="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(0,0,0,100);
        //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.beginPath();
        context.moveTo(0,0);
        context.lineTo(50,0);
        context.lineTo(100,50);
        context.lineTo(50,100);
        context.lineTo(0,100);
        context.lineTo(0,0);
        context.stroke();
        context.fill();
        context.closePath();
        
    
    }
}
$(document).ready(function(e) {
    
});

</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>