JSFiddle - React, Tailwind, and code Playground

HTML

<div id="central">
  <div id="floater_body"></div>
  <div id="body"></div>
</div>

CSS

#central{width:100%; height:500px; background-color:red;}
#floater_body{
	height: 100px;
}
#body{width:200px; height:100px;background-color:#000; margin-right: auto;
	margin-left: auto;}

JavaScript

var flexsize = $("#central").width();
	
	$('#floater_body').animate(function(){
		marginBottom: flexsize
	});