为什么是这个样子的呢？

 window.onload = function(){

        var canvas = document.getElementById("canvas");

        canvas.width = 800;

        canvas.height = 800;

        var context=canvas.getContext("2d");

var LinearGrad=context.createLinearGradient(0,0,800,800);

LinearGrad.addColorStop(0.0,'#fff');

LinearGrad.addColorStop(1.0,'#000');

context.fillStyle=LinearGrad;

context.fillRect(0,0,400,400);

}