Problem with HTML preview

You can use iframe for this. This way, if the user modifies CSS, it will not compromise the main document.

Following basic code:

JavaScript:

function preview(input) {
  const ta = document.querySelector("#ta2")
  var preview = document.getElementById("preview")

  ta.value = input.value

  preview.contentWindow.document.open()
  preview.contentWindow.document.write(ta.value)
  preview.contentWindow.document.close()
}

CSS:

body {
  font-family: Arial, Helvetica, sans-serif;
}

.container {
  position: fixed;
  top: 0;
  left: 0;
  width: calc(100% - 50px);
  height: 100%;
  padding: 25px;
}

.container-horizontal {
  display: flex;
  flex-direction: row;
  border: 1px solid silver;
  overflow: hidden;
  xtouch-action: none;
  height: calc(100% - 100px);
  box-shadow: 0 2px 4px 0 rgba(0, 0, 0, 0.16), 0 2px 10px 0 rgba(0, 0, 0, 0.12);
  position: relative;
}

.container-vertical {
  display: none;
  flex-direction: column;
  height: calc(100% - 100px);
  border: 1px solid silver;
  overflow: hidden;
  box-shadow: 0 2px 4px 0 rgba(0, 0, 0, 0.16), 0 2px 10px 0 rgba(0, 0, 0, 0.12);
  position: relative;
}

.esquerda {
  flex: 0 0 auto;
  /* only manually resize */
  width: 50%;
  min-height: 100%;
  min-width: 150px;
  max-width: 80%;
  white-space: nowrap;
  background: #eee;
  color: white;
  padding: 10px;
}

.direita {
  border: none;
  flex: 1 1 auto;
  /* resizable */
  padding: 10px;
  width: 10%;
  min-height: 100%;
  background: #fff;
  word-wrap: break-word;
  overflow: auto;
}

.topo {
  flex: 0 0 auto;
  /* only manually resize */
  padding: 10px;
  height: 45%;
  min-height: 10%;
  max-height: 80%;
  width: 100%;
  white-space: nowrap;
  background: #eee;
  color: white;
}

.base {
  flex: 1 1 auto;
  /* resizable */
  padding: 10px;
  height: 10%;
  width: calc(100% - 20px);
  background: #fff;
  word-wrap: break-word;
  overflow: auto;
}

.divisor {
  flex: 0 0 auto;
  width: 10px;
  background-color: #eee;
  border: 2px solid #ddd;
  cursor: col-resize;
}

.divisor-horizontal {
  flex: 0 0 auto;
  height: 10px;
  background-color: #eee;
  border: 2px solid #ddd;
  cursor: row-resize;
}

.ta {
  display: block;
  width: 100%;
  height: 100%;
  padding: 0;
  margin: 0;
  background: #eee;
  color: #000;
  resize: none;
  border: none;
  outline: none;
}

.botao {
  display: inline-block;
  height: 100%;
  background: #0084FF;
  color: #fff;
  border: none;
  border-radius: 3px;
  margin-left: 8px;
  padding: 10px;
  font-size: 20px;
  cursor: pointer;
  float: left;
}

.botao-e {
  display: inline-block;
  height: 100%;
  background: #0084FF;
  color: #fff;
  border: none;
  border-radius: 3px;
  margin-right: 8px;
  margin-left: 8px;
  padding: 10px;
  cursor: pointer;
}

.botao-e i {
  font-size: 20px;
}

.barra {
  padding: 8px 0;
  height: 40px;
  width: 100%;
  background-color: #ddd;
}

.float-right {
  float: right;
}

.labelbarra {
  padding: 13px;
  font-weight: bold;
  color: #555;
}

.inputbarra {
  font-size: 20px;
  padding: 8px;
  width: 140px;
  text-align: center;
  border: 1px solid #ccc;
  border-radius: 6px;
}

.pre-full {
  position: fixed;
  display: block;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  z-index: 9999;
  background-color: #fff;
}

.botao-flutuante {
  position: absolute;
  top: 5px;
  right: 5px;
  width: 30px;
  height: 30px;
  background: #0084FF;
  color: #fff;
  border: none;
  border-radius: 3px;
  font-size: 20px;
  cursor: pointer;
  padding: 5px;
}

.content:before {
  content: '\e813';
}


/* ---------------------------------------------- */

.sw-tog {
  position: relative;
  display: inline-block;
  width: 66px;
  height: 40px;
  float: left;
  margin-left: 8px;
}

.sw-top input {
  display: none;
}

.sli-tog {
  position: absolute;
  cursor: pointer;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: #bbb;
  -webkit-transition: .4s;
  transition: .4s;
  border-radius: 3px;
}

.sli-tog:before {
  position: absolute;
  content: "";
  height: 28px;
  width: 28px;
  left: 6.5px;
  bottom: 6.5px;
  background-color: #fff;
  -webkit-transition: .4s;
  border-radius: 3px;
  transition: .4s;
}

.tog:checked+.sli-tog {
  background-color: #0084FF;
}

.tog:focus+.sli-tog {
  box-shadow: 0 0 1px #0084FF;
}

.tog:checked+.sli-tog:before {
  -webkit-transform: translateX(26px);
  -ms-transform: translateX(26px);
  transform: translateX(26px);
}

#tempo-real {
  display: inline-block;
  margin: 12px;
  float: left;
  font-weight: bold;
  font-family: 'Roboto', sans-serif;
  color: #777;
  -webkit-touch-callout: none;
  -webkit-user-select: none;
  -khtml-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

HTML:

<div class="container" id="container">
  <div class="container-horizontal">
    <div class="esquerda">
      <textarea id="ta2" class="ta" oninput="preview(this)"></textarea>
    </div>
    <div class="divisor">
    </div>
    <button onclick="fullpage()" class="botao-flutuante"><i id="expand" class="fas fa-expand-arrows-alt"></i></button>
    <iframe id="preview" class="direita" src="about:blank"></iframe>
  </div>
</div>

Demo