insertHTML destroyed html in selctedtext

Hi.

I want create a plugin to style text. Its working, but its remove the html in selected text too. (This here is only a very simple example)

here the textarea html content:

<strong>Initial value</strong>

here the code snippet:
(getSelectionHTML taken from viewtopic.php?p=31688#p31688)

function getSelectionHTML(selection)
{
   var range = (document.all ? selection.createRange() : selection.getRangeAt(selection.rangeCount - 1).cloneRange());
   if (document.all)
   {
      return range.htmlText;
   }
   else
   {
      var clonedSelection = range.cloneContents();
      var div = document.createElement('div');
      div.appendChild(clonedSelection);
      return div.innerHTML;
   }
}

var mySelection = editor.getSelection();
var selectedText = getSelectionHTML(mySelection.getNative());
editor.insertHtml('<span style="color:red;">'+selectedText+'</span>');

html content after execute code:

<strong>Initial </strong><span style="color:red;">value</span><strong>.</strong>

"value" is not <strong> anymore. What is my mistake?