代码不通过

  window.onload = function(){

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

        canvas.width = 800;
        canvas.height = 800;

        var cxt=canvas.getContext('2d');
        cxt.moveTo(10,10);
        cxt.lineTo(100,400);
        cxt.lineWidth=3;
        cxt.strokeStyle="rgba(0,0,0,0.5)";
        cxt.stroke();

    }