JSFiddle - React, Tailwind, and code Playground

HTML

<div class="universe">
  <div class="somewidth">
      200px
  </div>
  <div class="everythingelse">
      rest of the screen
  </div>
</div>

CSS

.universe {
  width: 100%;
  height: 400px;
}
.universe > div { float: left; color: white }

.somewidth {
  width: 200px;
  height: 100%;
  background-color: brown;
}

.everythingelse {
  width: 800px; /* fallback for emergencies */
  width: calc(100% - 200px);
  width: -moz-calc(100% - 200px);
  width: -webkit-calc(100% - 200px);
  height: 100%;
  background-color: lightblue;
}