JSFiddle - React, Tailwind, and code Playground

HTML

<div class=m>
  <div class=l_1>left_______________________________________________X</div>
  <div class=r_1>right</div>
</div>

CSS

.m {
  display: flex;
  flex-wrap: wrap;
}

.l_1 {
  background-color: red;
  flex: 1;
}

.r_1 {
  background-color: yellow;
  flex: 1;
}