RT:
#container {
width: 700px;
height: 300px;
outline: 1px solid black;
display: flex;
justify-content: center;
align-items: center;
}
#container>div {
width: 650px;
height: 10px;
background-image: repeating-linear-gradient(90deg, transparent, transparent 10px, #fff 0, #fff 20px),
linear-gradient(to right, white, black 20% 80%, white);
}