JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
    <div class="one"></div>
    <div class="two"></div>
</div>

CSS

.wrapper {
    width: 200px;
    background:gray;
    display: inline-block;
    height:20px;
}
.one{
    width:50px;
    background:red;
    height: 20px;
    float:left;
}
.two{
    width:-moz-calc(100% - .one);
    background:black;
    height:20px;
}