JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
        <div class="left">
            fill free space (100% - right)
        </div>
        <div class="right">
            fixed width
        </div>
    </div>

CSS

.container {
            /* container don't matter */
            width: 500px;
            background-color: bisque;
            height: 50px;
        }

            .container .left {
               /* display: inline-block; */ 
                margin-right: 50px;
                background-color: burlywood;
                height: 50px;
            }

            .container .right {
                float: right;
                background-color: chartreuse;
                width: 50px;
                height: 50px;
            }