不知为什么只有一个a：hover img的效果出来了，而a：hover的阴影没有出来？
Because the attributes with transition effects do not include box-shadow
box-shadow
https://www.w3.org/TR/css3-transitions/#properties-from-css-
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Because the attributes with transition effects do not include
box-shadowhttps://www.w3.org/TR/css3-transitions/#properties-from-css-