JSFiddle - React, Tailwind, and code Playground

HTML

<div id="myElement">Hello everyone! guess Who</div>

JavaScript

HTMLElement.prototype.mark=function(Str=null){var T=this;
        if(Str === null){ 
            if(T.oV){ T.innerHTML=T.oV }
            return T
        }
        var V = T.oV = T.oV || T.innerText, 
        s = V.indexOf(Str), e = Str.length + s;
        V = V.substr(0,s) + `<span style="background-color:lightblue">`
        + V.substr(s,e - s + 1) + `</span>` + V.substr(e+1);
        T.innerHTML=V;
        return T;
    }
    
    var E = document.getElementById("myElement");
      E.mark("Hello");//finds the string and highlight
      // E.mark();// revert to original text