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<style type="text/css">
body {
    font-size: 12px;
}
.block { width:100px; background-color:#CCCCCC; }
.block a { display:block; height:20px; width:50px; line-height:20px; color:#000000; overflow:hidden;  }
.block a:hover { width:100%; overflow:visible; }</style>
<div class="block">
    <a href="#">内容啊内容啊内容啊内容啊内容啊内容啊</a>
</div>