JSFiddle - React, Tailwind, and code Playground

HTML

<body onload="initDoc();">
<form name="compForm" method="POST" onsubmit="if(validateMode()){this.myDoc.value=oDoc.innerHTML;return true;}return false;">
<div id="textBox" contenteditable="true" style="width:500px; height:150px; border:thin solid #000"></div>
<p id="editMode"><input type="checkbox" name="switchMode" id="switchBox" onchange="setDocMode(this.checked);" /> <label for="switchBox">Show HTML</label></p>
</form>
</body>

JavaScript

var oDoc, sDefTxt;

function initDoc() {
onload();
  oDoc = document.getElementById("textBox");
  sDefTxt = oDoc.innerHTML;
  if (document.compForm.switchMode.checked) { setDocMode(true); }
}

function validateMode() {
  if (!document.compForm.switchMode.checked) { return true ; }
  alert("Uncheck \"Show HTML\".");
  oDoc.focus();
  return false;
}

function setDocMode(bToSource) {
  var oContent;
  if (bToSource) {
    oContent = document.createTextNode(oDoc.innerHTML);
    oDoc.innerHTML = "";
    var oPre = document.createElement("pre");
    oDoc.contentEditable = false;
    oPre.id = "sourceText";
    oPre.contentEditable = true;
    oPre.appendChild(oContent);
    oDoc.appendChild(oPre);
  } else {
    if (document.all) {
      oDoc.innerHTML = oDoc.innerText;
    } else {
      oContent = document.createRange();
      oContent.selectNodeContents(oDoc.firstChild);
      oDoc.innerHTML = oContent.toString();
    }
    oDoc.contentEditable = true;
  }
  oDoc.focus();
}
function enterKeyPressHandler(evt) {
    var sel, range, br, addedBr = false;
    evt = evt || window.event;
    var charCode = evt.which || evt.keyCode;
    if (charCode == 13) {
        if (typeof window.getSelection != "undefined") {
            sel = window.getSelection();
            if (sel.getRangeAt && sel.rangeCount) {
                range = sel.getRangeAt(0);
                range.deleteContents();
                br = document.createElement("br");
                range.insertNode(br);
                range.setEndAfter(br);
                range.setStartAfter(br);
                sel.removeAllRanges();
                sel.addRange(range);
                addedBr = true;
            }
        } else if (typeof document.selection != "undefined") {
            sel = document.selection;
            if (sel.createRange) {
                range = sel.createRange();
                range.pasteHTML("<br>");
                range.select();
                addedBr = true;
            }
       ...