JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

body {
  background: black
}

div {
  width: 100px;
  height: 4px;
  background: linear-gradient(90deg, rgba(255,255,255,0) 0%, rgba(255,255,255,0.8) 35%, rgba(255,255,255,1) 50%, rgba(255,255,255,0.8) 65%, rgba(255,255,255,0) 100%);
}