SOURCE

<div class="a">
  <div class="b"></div>
</div>

.a {
  position: relative;
  width: 100px;
  height: 100px;
  border: 1px solid red;
}
.b {
  position: absolute;
  width: 50px;
  height: 50px;
  background: blue;
  animation: move 2s linear infinite;
}
@keyframes move {
  from { top: 100%; }
  to { top: 0%; }
}