Change format text selection in div

hi ;)

You forgive my English;)
I speak Spanish.

I want to put it into the selected text in bold tag DIV.
Most do not know how to write the script
Already tried unsuccessfully
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<style type="text/css">
.highlighted {
   background-color: yellow;
   color: darkgreen;
   font-weight: bold;
}

</style>

<script>

function decodeIt(textfield) {
strSelection = ""
if (document.selection) {
strSelection = document.selection.createRange().text;
strSelection = strSelection.replace(new RegExp("<","g"), "&lt;");
strSelection = strSelection.replace(new RegExp(">","g"), "&gt;");
document.selection.createRange().text = strSelection;
}
//MOZILLA/NETSCAPE support
else if (textfield.selectionStart || textfield.selectionStart == '0')
{
textfield.focus();
var startPos = textfield.selectionStart;
var endPos = textfield.selectionEnd;
strSelection = textfield.value.substring(startPos, endPos)
alert('hola '+strSelection)
strSelection = strSelection.replace(new RegExp("<","g"), "&lt;");
strSelection = strSelection.replace(new RegExp(">","g"), "&gt;");
textfield.value = textfield.value.substring(0, startPos) +
strSelection + textfield.value.substring(endPos,
textfield.value.length);
}
}

function m(){
element=document.getElementById('n');
sel = window.getSelection();
selectionStart= 2
selectionEnd= 4


   if (typeof document.createRange != 'undefined') {
     var range = document.createRange();
     if (element.firstChild) {
       range.setStart(element.firstChild, startOffset);
       range.setEnd(element.firstChild, endOffset);
       var span = document.createElement('span');
       span.className = 'highlighted';
       range.surroundContents(span);

     }
   }


}
</script>
<input type="button" onclick="decodeIt(document.y.i)" value="Decode
it">
<input type="button" onclick="m()" value="Decode it div">
<form name=y id=y>
<textarea id=i name=i >asoa dald aldada</textarea>
</form>
<div id=n name=n>strSelection asas</div>
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Thank you for your time ;)

angel matias orozco