JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
  <div class="leftSide">

  </div>
  <div class="mainCont">

  </div>
</div>

CSS

html,
body {
  width: 100%;
  height: 100%;
  margin: 0;
  padding;
  0;
}

.wrapper {
  width: 100%;
  height: 100%;
}

.leftSide {
  float: left;
  width: 200px;
  height: 100%;
  background: tan;
}

.mainCont {
  width: calc(100% - 200px);
  height: 100%;
  float: left;
  background: purple;
}