JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main_div">
    <div class="line">blabla</div>
    <div class="line">asdasd</div>   
</div>

CSS

#main_div{
    margin: 0 auto;
    width: 70%;
    border: 6px solid black;    
}
#main_div:after{
    content:'';
    display:block;
    width:100%;
    clear:both;
}

.line{
    float: left;
    border-bottom: 1px solid gray;
    width: 50%;
}