JSFiddle - React, Tailwind, and code Playground

HTML

<div>Hello World!</div><p>All your Foo bar are belong to us!</p>

JavaScript

var highlight = function(what){
   var html  = document.body.innerHTML,

       word  = "(" + what + ")",
       match = new RegExp(word, "gi");

   html = html.replace(match, "<span style='background-color: red'>$1</span>");

   document.body.innerHTML = html;
};

highlight('hello');