.container {
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
  width: 250px;
  height: 100px;
  align-content: flex-start;
  background:#00ffff;
}
.testcase {
  width: 100px;
  height: 50px;
}
.testcase div {
  width: 50px;
  height: 50px;
  position: relative;
}
.testcase>div {
  background: green;
}
.testcase>div>div {
  left: 50px;
  backdrop-filter: invert(1);
  background: transparent;
  border: 1px solid black;
}
* {
  box-sizing: border-box;
}