CKEditor

HTML

<script src="https://cdn.ckeditor.com/4.5.8/full/ckeditor.js"></script>
<textarea id="editor1"></textarea>
<button onclick="dummyContent()">Insert content</button>

JavaScript

CKEDITOR.replace('editor1', {
	entities_latin: false,
  //allowedContent: true,
  extraAllowedContent: 'div{border*,padding}',
});

dummyContent = function() {
	var html = '<div style="border:none;border-top:solid #E1E1E1 1.0pt">Dummy</div>';

	CKEDITOR.instances.editor1.insertHtml(html);
}