4.1 创建翻转器

其实就是切换资源，改变了元素的属性；

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="refresh" content="20">
    <link rel="stylesheet" href="myCSS.css">
</head>
<body>
<img id="myImageID" src="src/test_on.png" onmouseover="changeImageOver()" onmouseleave="changeImageLeave()">
<script>
    function changeImageOver() {
        document.images['myImageID'].src='src/test_on.png';
    }
    function changeImageLeave() {
        document.getElementById("myImageID").src = 'src/test_off.png';
    }
</script>
</body>
</html>