实现原理：变化前的图标和变化后的图标在一张图片上，用这张图片作为背景，通过定义背景的位置来实现显示哪个图标，其中还带着滑动的动画效果。
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<title>Document</title>
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.wh{width: 17px;height: 13px;float:left;margin-right:5px;}
.icon1{background: url(img/logo-sprite.png) no-repeat -1px -1px;transition: background 1s; }
.icon2{background: url(img/logo-sprite.png) no-repeat -29px -1px;transition: background 1s;}
.icon3{background: url(img/logo-sprite.png) no-repeat -59px -1px;transition: background 1s;}
.icon1:hover{background: url(img/logo-sprite.png) no-repeat -1px -18px;}
.icon2:hover{background: url(img/logo-sprite.png) no-repeat -29px -18px;}
.icon3:hover{background: url(img/logo-sprite.png) no-repeat -59px -18px;}
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