JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
  <div class="a">Block A</div>
  <div class="b">Block B</div>
</div>

CSS

.parent {
    background: #AAA;
    height: 240px;
    width: 320px;
}
.a {
    background: green;
    height: 20px;
}
.b {
    background: rgba(0,0,0,0.3);
    height: 100%;
}