JSFiddle - React, Tailwind, and code Playground

HTML

<div id="blanket" style="display:none;"></div>
	<div id="popUpDiv" style="display:none;">
    
    	<a href="#" id="closeLink" >Click to Close CSS Pop Up</a>
    </div>	
   <a id="link">Click to Open CSS Pop Up</a>

CSS

#blanket {
   background-color:#111;
   opacity: 0.65;
   *background:none;
   position:absolute;
   z-index: 9001;
   top:0px;
   left:0px;
   width:100%;
}

#popUpDiv {
	position:absolute;
	background:url(pop-back.jpg) no-repeat;
	width:400px;
	height:400px;
	border:5px solid #000;
	z-index: 9002;
}

#popUpDiv { a position:relative; top:20px; left:20px}

JavaScript

$(document).ready( function() {
    $("#link,#closeLink").click( function () { popup('popUpDiv')});
});

function toggle(div_id) {
	var el = document.getElementById(div_id);
	if ( el.style.display == 'none' ) {	el.style.display = 'block';}
	else {el.style.display = 'none';}
}
function blanket_size(popUpDivVar) {
	if (typeof window.innerWidth != 'undefined') {
		viewportheight = window.innerHeight;
	} else {
		viewportheight = document.documentElement.clientHeight;
	}
	if ((viewportheight > document.body.parentNode.scrollHeight) && (viewportheight > document.body.parentNode.clientHeight)) {
		blanket_height = viewportheight;
	} else {
		if (document.body.parentNode.clientHeight > document.body.parentNode.scrollHeight) {
			blanket_height = document.body.parentNode.clientHeight;
		} else {
			blanket_height = document.body.parentNode.scrollHeight;
		}
	}
	var blanket = document.getElementById('blanket');
	blanket.style.height = blanket_height + 'px';
	var popUpDiv = document.getElementById(popUpDivVar);
	popUpDiv_height=blanket_height/2-200;//200 is half popup's height
	popUpDiv.style.top = popUpDiv_height + 'px';
}
function window_pos(popUpDivVar) {
	if (typeof window.innerWidth != 'undefined') {
		viewportwidth = window.innerHeight;
	} else {
		viewportwidth = document.documentElement.clientHeight;
	}
	if ((viewportwidth > document.body.parentNode.scrollWidth) && (viewportwidth > document.body.parentNode.clientWidth)) {
		window_width = viewportwidth;
	} else {
		if (document.body.parentNode.clientWidth > document.body.parentNode.scrollWidth) {
			window_width = document.body.parentNode.clientWidth;
		} else {
			window_width = document.body.parentNode.scrollWidth;
		}
	}
	var popUpDiv = document.getElementById(popUpDivVar);
	window_width=window_width/2-200;//200 is half popup's width
	popUpDiv.style.left = window_width + 'px';
}
function popup(windowname) {
	blanket_size(windowname);
	window_pos(windowname);
	toggle('blanket');
	toggle(windowname);		
}