JSFiddle - React, Tailwind, and code Playground

HTML

<div id="redone"></div>
<div id="blackone"></div>

CSS

@keyframes filleffect
{
from {width:0;}
to {width: 400px;}
}

@-webkit-keyframes filleffect /* Safari and Chrome */
{
from {width:0;}
to {width: 400px;}
}
#redone
{
    animation:filleffect 5s;
    -webkit-animation:filleffect 5s;
    position:absolute;
    top:8px;
    left:8px;
    height:30px;
    background-color:red;
}
#blackone
{
    width:400px;
    height:30px;
    background-color:#000;
}