好的,这是我使用纯 CSS 进行的非常粗略的尝试。
这是小提琴:http: //jsfiddle.net/PJbhQ/4/
这是CSS:
.nav-tabs > li > a{
    box-shadow: -2px -1px 3px -1px #aeaeae, 2px -1px 3px -1px #aeaeae;
    background-image:linear-gradient(to bottom, #fefefe, #dddedd);
    //border-bottom-color: transparent;
    border-radius: 8px 20px 0 0;
    border-bottom: none;
}
.nav-tabs > li > a:after{
    background-attachment: scroll;
    background-clip: border-box;
    background-color: transparent;
    background-image: radial-gradient(circle at 100% 0 , rgba(255, 255, 255, 0) 14px, #999 17px, #dddedd 18px);
    background-origin: padding-box;
    background-position: left bottom, right bottom, right top, left top;
    background-repeat: no-repeat;
    background-size: 100% 100%;
    content: "null";
    color:rgba(0,0,0,0);
    height: 20px;
    left: 31px;
    position: relative;
    top: 8px;
    width: 20px;
    z-index: 5;
}
.nav-tabs > li.active > a {
    box-shadow: -2px -1px 3px -1px #aeaeae, 2px -1px 3px -1px #aeaeae;
    color: @gray;
    background: #ffffff;
    border-bottom: none;
}   
.nav-tabs > li.active > a:after{
    background-attachment: scroll;
    background-clip: border-box;
    background-color: transparent;
    background-image: radial-gradient(circle at 100% 0 , rgba(255, 255, 255, 0) 14px, #999 17px, #fff 18px);
    background-origin: padding-box;
    background-position: left bottom, right bottom, right top, left top;
    background-repeat: no-repeat;
    background-size: 100% 100%;
}
(HTML和JS是一样的)
从Inset border-radius with CSS3中可以看出,我对凸曲线使用了渐变
希望这可以帮助!:)