JSFiddle - React, Tailwind, and code Playground

HTML

<div class="main">
<div class="header" id= "headerxx">
    </div>

    <div class="mainbody">
    </div>
    <div class="clear"></div>
</div>

CSS

body {
    background-color: #F0F0F0; 
    margin-top: 0px;
    width: 100%;
    margin-left: 0px;
    }

  .header {
    width: 10%;
    height: 200px;
    margin-top: 0px;
    float: left;
    background-color: #000000;
 }

.mainbody {
    width: 90%;
    height: 200px;
    float: left;
    background-color: #666666;
 }

.clear {
    clear: both;
}