JSFiddle - React, Tailwind, and code Playground

HTML

<div class="leftfixed">
box left fixed width 1
</div>

<div class="right">
box right
</div>

<div class="leftfixed">
box left fixed width 2
</div>

CSS

.leftfixed {
  float:left;
  width:200px;
  background:purple;
  height:100px;
}

.right {
  margin-left:220px;
  background:yellow;
  height:200px;
  }

@media (max-width:500px) {
  .leftfixed {
    float:none;
      width:100%;
      background:blue;
   }
   
   .right {
     margin-left:0;
     
     }
}