JSFiddle - React, Tailwind, and code Playground

HTML

<div id="parent_div">
	<div id="left_div">A</div>
	<div id="right_div">
		<input type="button" value="ABC">
	</div>
</div>

CSS

#parent_div {
	width: 100%;
	height: 10%;

        position: relative;
        margin-right: 305px;
        min-width: 40px;
    
}

#left_div {
	width: 80%;
	min-width: 100px;
	height: 80%;
	float: left;
	background-color: #000;
    color: #FFF;
}

#right_div {
	width: 15%;
	min-width: 100px;
	float: right;
	background-color: blue;
     position:absolute;
        right: 0px;
}

input[type=button] {
	font-size: 2rem;
}