JSFiddle - React, Tailwind, and code Playground

HTML

<div>div</div>
<div style="z-index:2;">div</div>
<div>div</div>

CSS

.largest{
    color:red;
}

JavaScript

var elems = document.body.getElementsByTagName("*");
var largest;
var check = 0;
for(i=0;i<elems.length;i++){
    if(elems[i].style.zIndex > check){
        check = elems[i].style.zIndex;
        largest = elems[i];
    }
}
largest.className = "largest";