<div class="container">
...
<div class="container__side container__side--left"></div>
<div class="container__triangle container__triangle--left"></div>
<div class="container__triangle container__triangle--right"></div>
<div class="container__side container__side--right"></div>
</div>
.container {
align-items: center;
display: flex;
justify-content: center;
background-color: #bbb;
height: 32px;
position: relative;
}
.container__side {
bottom: -8px;
position: absolute;
z-index: -1;
border: 16px solid #ccc;
border-left-color: transparent;
}
.container__side--left {
left: -24px;
}
.container__side--right {
right: -24px;
}
.container__triangle {
position: absolute;
top: 100%;
border: 8px solid transparent;
border-bottom-width: 0;
border-top-color: #aaa;
}
.container__triangle--left {
border-right-width: 0;
left: 0;
}
.container__triangle--right {
border-left-width: 0;
right: 0;
}