<div></div>

body {
    margin: 0;
}

div {
    position: relative;
    height: 50px;
    margin-top: 50px;

    background: #000;
}

div::before {
    content: "";
    position: absolute;
    top: -50px;
    left: 0;

    width: 100%;

    border-left: 100vw solid #999;
    border-top: 50px solid transparent;
    border-bottom: 50px solid transparent;
    opacity: 0.5;
}