Insert an HTML element in a contentEditable element

Here is a kickstart

// get the selection range (or cursor     position)
var range = window.getSelection().getRangeAt(0); 
// create a span
var newElement = document.createElement('span');
newElement.id = 'myId';
newElement.innerHTML = 'Hello World!';

// if the range is in #myDiv ;)
if(range.startContainer.parentNode.id==='myDiv') {
   // delete whatever is on the range
   range.deleteContents();
   // place your span
   range.insertNode(newElement);
}

I don't have IE but works fine on firefox, chrome and safari. Maybe you want to play with range.startContainer to proceed only if the selection is made on the contentEditable div.

EDIT: According to quirksmode range intro you have to change the window.getSelection() part to be IE compatible.

var userSelection;
if (window.getSelection) {
    userSelection = window.getSelection();
}
else if (document.selection) { // should come last; Opera!
    userSelection = document.selection.createRange();
}

The following will do this in all major browsers (including IE 6). It will also handle cases where the end of the selection is outside your <div> and cases where the selection is contained within a child (or more deeply nested) element inside the <div>.

2019 addendum: The second branch of insertNodeOverSelection is for IE <= 8 only and could be removed now.

function isOrContainsNode(ancestor, descendant) {
    var node = descendant;
    while (node) {
        if (node === ancestor) return true;
        node = node.parentNode;
    }
    return false;
}

function insertNodeOverSelection(node, containerNode) {
    var sel, range, html;
    if (window.getSelection) {
        sel = window.getSelection();
        if (sel.getRangeAt && sel.rangeCount) {
            range = sel.getRangeAt(0);
            if (isOrContainsNode(containerNode, range.commonAncestorContainer)) {
                range.deleteContents();
                range.insertNode(node);
            } else {
                containerNode.appendChild(node);
            }
        }
    } else if (document.selection && document.selection.createRange) {
        range = document.selection.createRange();
        if (isOrContainsNode(containerNode, range.parentElement())) {
            html = (node.nodeType == 3) ? node.data : node.outerHTML;
            range.pasteHTML(html);
        } else {
            containerNode.appendChild(node);
        }
    }
}
<input type="button" onmousedown="insertNodeOverSelection(document.createTextNode('[NODE]'), document.getElementById('test'));" value="insert">

<div contenteditable="true">
    <div id="test" style="background-color: lightgoldenrodyellow">
        This is the editable element where the insertion will happen. Select something or place the cursor in here, then hit the button above
    </div>
    <div>
        No insertion will happen here
    </div>
</div>