SOURCE

<div class="tab-bar">
    <div class="tab-bar-item">1</div>
    <div class="tab-bar-item">2</div>
    <div class="tab-bar-item">3</div>
    <div class="tab-bar-item">4</div>
    <div class="tab-bar-item">5</div>
</div>

body {
    background: #eee;
}
.tab-bar {
    position: fixed;
    left: 0;
    bottom: 0;
    z-index: 1;
    width: 100%;
    background: #fff;
    display: flex;
    justify-content: space-around;
    box-shadow: 0 0 5px #999;
}

.tab-bar::before,
.tab-bar-item {
    display: flex;
    width: 100px;
    height: 100px;
    align-items: center;
    justify-content: center;
}

.tab-bar::before {
    content: '';
    display: block;
    position: absolute;
    top: -20px;
    left: 0;
    right: 0;
    z-index: -1;
    margin: auto;
    background: #fff;
    border-radius: 50% 50% 0 0;
    box-shadow: 0 0 5px #999;
}

.tab-bar::after {
    content: '';
    display: block;
    position: absolute;
    top: 0;
    left: 0;
    z-index: -1;
    width: 100%;
    height: 100%;
    background: #fff;
}