JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wrapper">
    <div id="a">Hover over this div to see a cool tool tip!</div>
</div>

CSS

#a{
    background-color:yellow;
    padding:10px;
    border:2px solid red;    
}

.tooltip{
    background:black;
    color:white;
    padding:5px;
    box-shadow:0 0 10px 0 rgba(0, 0, 0, 1);
    border-radius:10px;
    opacity:0;
}

JavaScript

var div = document.getElementById('wrapper');
var a = document.getElementById("a");
var fadeSpeed = 25; // a value between 1 and 1000 where 1000 will take 10
                    // seconds to fade in and out and 1 will take 0.01 sec.
var tipMessage = "The content <br /> dof the tooltip...";

var showTip = function(){    
    var tip = document.createElement("span");
    tip.className = "tooltip";
    tip.id = "tip";
    tip.innerHTML = tipMessage;
    div.appendChild(tip);
    tip.style.opacity="0"; // to start with...
    var intId = setInterval(function(){
        newOpacity = parseFloat(tip.style.opacity)+0.1;
        tip.style.opacity = newOpacity.toString();
        if(tip.style.opacity == "1"){
            clearInterval(intId);
        }
    }, fadeSpeed);
};
var hideTip = function(){
    var tip = document.getElementById("tip");
    var intId = setInterval(function(){
        newOpacity = parseFloat(tip.style.opacity)-0.1;
        tip.style.opacity = newOpacity.toString();
        if(tip.style.opacity == "0"){
            clearInterval(intId);
            tip.remove();
        }
    }, fadeSpeed);
    tip.remove();
};

a.addEventListener("mouseover", showTip, false);
a.addEventListener("mouseout", hideTip, false);