JSFiddle - React, Tailwind, and code Playground

HTML

<div class="header">
    <div class="headerizquierda">IZQUIERDA</div>
    <div class="headerderecha">DERECHA</div>
</div>

CSS

.header {
width:100%;
background: #E1E1E1;
margin: 0px 0 1px;
padding: 0px 0px;
/* rounded corner */
-webkit-border-radius: 0px;
-moz-border-radius: 0px;
border-radius: 0px;
/* box shadow */
-webkit-box-shadow: 0 1px 3px rgba(0,0,0,.4);
-moz-box-shadow: 0 1px 3px rgba(0,0,0,.4);
box-shadow: 0 1px 3px rgba(0,0,0,.4);
 height: 1%; overflow: hidden;
}
.headerizquierda {
background-color:#E1E1E1;
float: left;
}
.headerderecha {
background-color:#E1E1E1;
float: right;
}