JSFiddle - React, Tailwind, and code Playground

HTML

<div class="main">
  <div class="first">
    FIRST
  </div>
  <div class="second">
    SECOND
  </div>
</div>

CSS

.main {
  display: flex;
}

.first {
  flex: 0;
  background-color: red;
  height: 20px;
}

.second {
  flex: 4;
  background-color: green;
  height: 20px;
}