html2canvas 适用于 jsfiddle 但不适用于我的网站

html2canvas([document.getElementById('mydiv')], {
    onrendered: function (canvas) {
        document.getElementById('canvas').appendChild(canvas);
        var data = canvas.toDataURL('image/png');
        var image = new Image();
        image.src = data;
        document.getElementById('image').appendChild(image);
    }
});

#mydiv {
    background-color: lightblue;
    width: 100px;
    height: 100px
}

<div id="mydiv"><p>text!</p></div>
<br><br>
 <div id="canvas"><p>Canvas:</p></div>
 <div id="image"><p>Image:</p></div>