外圆角选项卡

<!DOCTYPE html>
<html>

    <head>
        <meta charset="UTF-8">
        <title></title>
        <style type="text/css">
            .tab-x {
                display: flex;
                flex-direction: row-reverse;
                justify-content: flex-end;
                padding-left: 20px;
                border-bottom: 1px solid rgba(0, 0, 0, .1);
            }
            
            .tab-a {
                --backgroundColor: #fff;
                background-color: var(--backgroundColor);
                line-height: 20px;
                padding: 10px 20px;
                border-radius: 16px 16px 0 0;
                filter: drop-shadow(0 -1px 1px rgba(0, 0, 0, .1));
                position: relative;
            }
            /* 创建外侧圆弧 */
            
            .tab-a::before,
            .tab-a::after {
                content: '';
                position: absolute;
                bottom: 0;
                 16px;
                height: 16px;
            }
            
            .tab-a::before {
                background: radial-gradient(circle at 0 0, transparent, transparent 16px, var(--backgroundColor) 17px);
                right: 100%;
            }
            
            .tab-a::after {
                background: radial-gradient(circle at 100% 0, transparent, transparent 16px, var(--backgroundColor) 17px);
                left: 100%;
            }
            
            .tab-a.active {
                --backgroundColor: teal;
                color: #fff;
                z-index: 1;
            }
        </style>
    </head>

    <body>
        <div class="tab-x">
            <a href="javascript:" class="tab-a">选项卡3</a>
            <a href="javascript:" class="tab-a">选项卡2</a>
            <a href="javascript:" class="tab-a active">选项卡1</a>
        </div>
    </body>

</html>
效果图：