selectionStart and selectionEnd in contenteditable element

function GetSelectedText() {

            if (document.getSelection) {    // all browsers, except IE before version 9
                var sel = document.getSelection();
                    // sel is a string in Firefox and Opera, 
                    // and a selectionRange object in Google Chrome, Safari and IE from version 9
                    // the alert method displays the result of the toString method of the passed object
                alert(sel);
            } 
            else {
                if (document.selection) {   // Internet Explorer before version 9
                    var textRange = document.selection.createRange();
                    alert(textRange.text);
                }
            }
        }

<div>Test Example Microsoft T-shirt box</div>
<button onClick="GetSelectedText()">Get text</button>