JSFiddle - React, Tailwind, and code Playground

HTML

<div id="nav">
  <ul>
    <li id="tabOne" class="first current"><a href="./CS1.html" target="SheetView">Page One</a></li>
    <li id="tabTwo"><a href="./CS2.html" target="SheetView">Page Two</a></li>
      <li id="tabThree"><a href="./CS3.html" target="SheetView">Page Three</a></li>
    <li id="tabFour"><a href="./CS4.html" target="SheetView">Page Four</a></li>
    <li id="tabFive"><a href="./CS5.html" target="SheetView">Page Five</a></li>
    <li id="tabSix"><a href="./CS6.html" target="SheetView">Page Six</a></li>
  </ul>
</div>

CSS

.current{ background-color: #AAA; }

JavaScript

function toggleNavSelected(el){
    var list = document.getElementById('nav').children[0];
    for(var i=0; i<list.children.length; i++){
        var cur = list.children[i];
        if(el==cur){
            cur.classList.add("current");
        } else {
            if(cur.classList.contains("current")){
                cur.classList.remove("current");
            }
        }
    }
}

var list = document.getElementById('nav').children[0];
for(var i=0; i<list.children.length; i++){
    var el = list.children[i];
    el.firstChild.onclick = (function(){
        toggleNavSelected(this.parentElement);
        return false;
    });
}