textarea autosize on css+js

textarea-autosize.css

.custom-css-textarea {  
  width: 100%;
  min-height: 150px;
  box-sizing: border-box;
  /* box-sizing: padding-box; */
  overflow:hidden;
  /* stylization */
  padding:5px;  
}

textarea-autosize.html

<textarea class="custom-css-textarea" name="css" placeholder="Свои правила CSS"></textarea>

textarea-autosize.js

var textarea = document.querySelector('textarea');

textarea.addEventListener('keydown', autosize);
             
function autosize(){
  var el = this;
  setTimeout(function(){
    el.style.cssText = 'height:auto; padding:5px';
    // for box-sizing other than "content-box" use:
    // el.style.cssText = '-moz-box-sizing:content-box';
    el.style.cssText = 'height:' + el.scrollHeight + 'px';
  },0);
}