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<h1>鼠标经过灰色区域出现黑色三角</h1>
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div{ display: block; height: 30px; background:#ccc; }
div a{ display: block; width:50px; height: 30px; position: relative; }
div:hover a:after{
content:‘‘;
display:block;
border-style:solid;
border-width:0px 6px 6px 6px;
border-color:#000 transparent;
position:absolute;
left:50%;
bottom:0;
margin-left:-6px;
}
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原文：https://www.cnblogs.com/lstrive/p/9038419.html