JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wr">
    <div id="con_top"></div>
    <div id="con_bottom"></div>
</div>

CSS

#wr {
    height:400px;
    width:80%;
    margin:50px auto;
    border:1px solid black;
    position: relative;
}
    #con_top {
        height:40px;
        margin:5px;
        border:1px solid red;            
    }
    #con_bottom {
        border:1px solid blue;
        margin:5px;    
        position: absolute;
        top: 45px;
        bottom: 0;
        left: 0;
        right: 0;
    }