JSFiddle - React, Tailwind, and code Playground

HTML

<div id="block-new40">&nbsp;</div>

CSS

#block-new40{
    background: red;
    width: 100px;
    height: 100px;
    opacity: .5;
}

JavaScript

function setOpacity (myElement, opacityValue) {
    if (window.ActiveXObject) {
        myElement.style.filter = "alpha(opacity="
             + opacityValue*100 + ")"; // IE
    } else {
        myElement.style.opacity = opacityValue; // Gecko/Opera
    }
}
var elem = document.getElementById("block-new40");
elem.onmouseenter = function() {
    setOpacity (this, 1);
}
elem.onmouseleave = function() {
    setOpacity (this, 0.5);
}