JSFiddle - React, Tailwind, and code Playground

HTML

<div id="mydiv">HEllo</div>

CSS

#mydiv {
    z-index: 9999 !important;
    
}

JavaScript

console.log(jQuery("#mydiv").css("z-index"));
jQuery("#mydiv").css({'z-index':'3000 !important'});

console.log(jQuery("#mydiv").css("z-index"));