JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">

<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type" />
<title>Untitled 1</title>

</head>

<body>

<div id="cont">
<div id="left-box"></div>
<div id="top-rel-container">

<div id="head-top-thin">

</div>

<div id="head-bottom-thin">

</div>

</div>

</div>
</body>

</html>

CSS

#left-box{
    width:190px;
    height:90px;
    background-color:blue; 
}

#top-rel-container{
    float:left;
    width:400px;
    height:90px;
}
#head-top-thin{
    float:left;
    height:45px;
    background-color:red;
    width:400px;
}
#head-bottom-thin{
    float:left;
        width:400px;
    height:45px;
    background-color:green;
}

#cont{
    width:100%;
    height:90px;
    float:left;
}