Textarea, которая может выполнять подсветку синтаксиса на лету?

var $container = $('.container');
var $backdrop = $('.backdrop');
var $highlights = $('.highlights');
var $textarea = $('textarea');
var $toggle = $('button');


var ua = window.navigator.userAgent.toLowerCase();
var isIE = !!ua.match(/msie|trident\/7|edge/);
var isWinPhone = ua.indexOf('windows phone') !== -1;
var isIOS = !isWinPhone && !!ua.match(/ipad|iphone|ipod/);

function applyHighlights(text) {
  text = text
    .replace(/\n$/g, '\n\n')
    .replace(/[A-Z].*?\b/g, '<mark>$&</mark>');

  if (isIE) {
    // IE wraps whitespace differently in a div vs textarea, this fixes it
    text = text.replace(/ /g, ' <wbr>');
  }

  return text;
}

function handleInput() {
  var text = $textarea.val();
  var highlightedText = applyHighlights(text);
  $highlights.html(highlightedText);
}

function handleScroll() {
  var scrollTop = $textarea.scrollTop();
  $backdrop.scrollTop(scrollTop);

  var scrollLeft = $textarea.scrollLeft();
  $backdrop.scrollLeft(scrollLeft);
}

function fixIOS() {
  $highlights.css({
    'padding-left': '+=3px',
    'padding-right': '+=3px'
  });
}

function bindEvents() {
  $textarea.on({
    'input': handleInput,
    'scroll': handleScroll
  });
}

if (isIOS) {
  fixIOS();
}

bindEvents();
handleInput();

@import url(https://fonts.googleapis.com/css?family=Open+Sans);
*,
*::before,
*::after {
  box-sizing: border-box;
}

body {
  margin: 30px;
  background-color: #fff;
  caret-color: #000;
}

.container,
.backdrop,
textarea {
  width: 460px;
  height: 180px;
}

.highlights,
textarea {
  padding: 10px;
  font: 20px/28px 'Open Sans', sans-serif;
  letter-spacing: 1px;
}

.container {
  display: block;
  margin: 0 auto;
  transform: translateZ(0);
  -webkit-text-size-adjust: none;
}

.backdrop {
  position: absolute;
  z-index: 1;
  border: 2px solid #685972;
  background-color: #fff;
  overflow: auto;
  pointer-events: none;
  transition: transform 1s;
}

.highlights {
  white-space: pre-wrap;
  word-wrap: break-word;
  color: #000;
}

textarea {
  display: block;
  position: absolute;
  z-index: 2;
  margin: 0;
  border: 2px solid #74637f;
  border-radius: 0;
  color: transparent;
  background-color: transparent;
  overflow: auto;
  resize: none;
  transition: transform 1s;
}

mark {
  border-radius: 3px;
  color: red;
  background-color: transparent;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="container">
  <div class="backdrop">
    <div class="highlights"></div>
  </div>
  <textarea>All capitalized Words will be highlighted. Try Typing to see how it Works</textarea>
</div>