*{ margin: 0; padding: 0;}
.box{ width: 0; height: 0; border:50px solid #f00; border-color:red transparent transparent transparent;}
.box2{width: 0; height: 0; border:50px solid #f00; border-color:red red transparent transparent; line-height: 0;}
.box3{ width:200px; height: 100px; border: 5px solid #2E8DED; margin-left:10px; position: relative;}
.box3 .arror{ position: absolute; top: 100px; left:30px; width: 50px; height: 50px;}
.a1{ border:30px solid #f00; display: block; border-color:#2E8DED transparent transparent transparent;}
.a2{ position: absolute; left: 0; top:-8px; border:30px solid #f00; display: block; border-color:#fff transparent transparent transparent;}
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效果：
