在框内水平移动一条垂直线

<div id="container"></div>

#container {
  width: 200px;
  height: 400px;
  border: 1px solid black;
  position: relative;
}

#line {
  width: 100%;
  height: 2px;
  background-color: black;
  position: absolute;
  top: 0;
  left: 0;
  animation: moveLine 5s linear infinite;
}

@keyframes moveLine {
  0% {
    left: 0;
  }
  100% {
    left: 100%;
  }
}

const container = document.getElementById('container');
const line = document.createElement('div');
line.id = 'line';
container.appendChild(line);