JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main">
  <div id="sidebar">DIV1
        <div id="sidebar_2">DIV1 Sub</div>
  </div>
  <div id="page-wrap">DIV2</div>
</div>

CSS

#main
{ 
    display: block;
    width: 100%;
}
#sidebar    
{
    background-color: Aqua;
    float: left;
    width: 80%;
}

#sidebar_2    
{
    background-color: Lime;
}

#page-wrap  
{
    background-color: Gray;
}

#sidebar_2 {
    position:absolute;
    left:0;
    right:0;
}