<style>
#container {
  position: relative;
}

#control {
  position: absolute;
  background-color: black;
  width: 20px;
  height: 0px;
}

#target {
  position: absolute;
  left: 20px;
  background-color: rgba(0, 128, 0, 0.25);
  width: 280px;
  height: 0px;
}

.block {
  padding-left: 30px;
  border-bottom: 1px solid black;
  box-sizing: border-box;
  width: 300px;
  height: 50px;
  font-size: 20px;
}
</style>

<div id="container">
  <div id="control"></div>
  <div id="target"></div>
</div>
<script src="..