SOURCE

<div class="popup-container">
    <div class="popup-body">内容</div>
    <div class="popup-arrow"></div>
</div>

.popup-container {
    position: absolute;
}

.popup-body {
    padding: 5px 10px;
    border-radius: 2px;
    font-size: 13px;
    color: #fff;
    background-color: rgb(75, 156, 223);
    box-shadow: 0 2px 4px rgba(0, 0, 0, .12)
}

.popup-arrow {
    position: absolute;
    width: 1rem;
    height: .5rem;
    bottom: 0;
    left: calc(50% - 0.5rem);
}

.popup-arrow::before,.popup-arrow::after {
    position: absolute;
    display: block;
    content: '';
    border-color: transparent;
    border-style: solid;
    box-sizing: border-box;
}

.popup-arrow::before {
    bottom: 0;
    border-width: .5rem .5rem 0;
}

.popup-arrow::after {
    bottom: -.5rem;
    border-width: .5rem .5rem 0;
    border-top-color: rgb(75, 156, 223);
}