JSFiddle - React, Tailwind, and code Playground

HTML

<section>
    <div class="box">13534534534534</div>
    <div class="box">233454354</div>
    <div class="box">34345345345</div>
    <div class="box">4345345</div>
</section>

CSS

section {
    display: -webkit-flex;
    display: flex;
    
    -webkit-flex-direction: row;
    flex-direction: row;
    
    -webkit-flex-wrap: wrap;
    flex-wrap: wrap;
    
    width: 100%;
    box-sizing:border-box;
    margin:0;
}

.box {
    border: 1px solid black;
    background: #ccc;
    display: block;
    
     /* 
        50% - (border-left-width + border-right-width +
               padding-left + padding-right)
    */
    -webkit-flex-basis: calc(50% - 2px);
    flex-basis: calc(50% - 2px);
    
    margin:0;
}