JSFiddle - React, Tailwind, and code Playground

HTML

<div id="t">asdf</div>

CSS

#t {
    z-index: 100;
}

JavaScript

var getHighestZIndex = function () {
    var highestZIndex = 0,
        zIndex,
        pos,
        tags = document.body.getElementsByTagName('*');
    
    for (var c = 0; c < tags.length; c++) {
        // Get the original 'position' property
        pos = getComputedStyle(tags[c]).position;
        // Set it temporarily to 'relative'
        tags[c].style.position = "relative";
        // Grab the z-index
        zIndex = getComputedStyle(tags[c]).zIndex;
        // Reset the 'position'
        tags[c].style.position = pos;
        
        console.log(tags[c], "zIndex=", zIndex);

        if (zIndex > highestZIndex) { highestZIndex = zIndex; }
    }

    return highestZIndex;
};

console.log(getHighestZIndex());