实现效果：
css实现梯形tab标签—jensonhui's blog

<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <div class="box">
        <ul>
            <li>aaaa</li>
            <li>bbbb</li>
            <li>cccc</li>
            <li>dddd</li>
        </ul>
    </div>
</body>
<style>
    *{
        padding: 0;
        margin: 0;
    }
    ul{
        margin:100px;
        height:20px;
        list-style:none;
        padding-left:20px;
    }
    li{
        text-align: center;
        width:50px;
        height:0;
        float: left;
        margin-left:-25px;
        border-width:0 20px 37px 27px;
        border-color:transparent transparent red;
        border-style: none solid solid;
        
    }   
    ul li:first-child{
        margin-left: 0;
    }
</style>
</html>