為何沒有直線出現(xiàn)？<canvas id="myCanvas" width="400" height="200">canvas {    border: 1px dashed black;}    window.onload = function() {        var canvas = document.getElementById("myCanvas");        var context = canvas.getContext("2d");        context.lineWidth = 10;        context.moveTo(0, 50);        context.lineTo(400, 150);        context.stroke();    }        分別用chrome firefox 打開jsfiddle,沒有直線出現(xiàn)？