Ribbon

HTML

<div class="ribbon">
    <!-- The left side -->
    <div class="ribbon__side ribbon__side--l"></div>

    <!-- The left triangle displayed below the content -->
    <div class="ribbon__triangle ribbon__triangle--l"></div>

    <!-- The right triangle displayed below the content -->
    <div class="ribbon__triangle ribbon__triangle--r"></div>

    <!-- The right side -->
    <div class="ribbon__side ribbon__side--r"></div>

    <!-- The content -->
    <div class="ribbon__content"></div>
</div>

CSS

.ribbon {
    /* Center the content */
    align-items: center;
    display: flex;
    justify-content: center;

    /* Size */
    height: 2rem;

    /* Use to position the corners */
    position: relative;
}

.ribbon__content {
    /* Background color */
    background-color: #9ca3af;
    z-index: 1;

    height: 100%;
    width: 100%;
}
.ribbon__side {
    bottom: -0.5rem;
    position: absolute;

    /* Displayed under the ribbon */
    z-index: 1;

    /* Background */
    border: 1rem solid #d1d5db;
}

.ribbon__side--l {
    /* Position */
    left: -1.5rem;
    border-color: #d1d5db #d1d5db #d1d5db transparent;
}

.ribbon__side--r {
    /* Position */
    right: -1.5rem;
    border-color: #d1d5db transparent #d1d5db #d1d5db;
}

.ribbon__triangle {
    position: absolute;
    top: 100%;

    border: 0.5rem solid transparent;
    border-bottom-width: 0;
    border-top-color: #aaa;
}

.ribbon__triangle--l {
    border-right-width: 0;
    left: 0;
}

.ribbon__triangle--r {
    border-left-width: 0;
    right: 0;
}