JSFiddle - React, Tailwind, and code Playground

HTML

<div id="a">
sup b
</div>
<div>
wefplew fplwe pflwe plfewpl few wefplew fplwe pflwe plfewpl few  wefplew fplwe pflwe plfewpl few  wefplew fplwe pflwe plfewpl few  wefplew fplwe pflwe plfewpl few 
</div>

CSS

#a
{
  background: red;
  position: fixed;
  width: 100%;
}