Как изменить стиль шрифта window.getSelection()

function surroundSelection() {
        var span = document.createElement("span");
        span.style.fontWeight = "bold";
        span.style.color = "black";

        if (window.getSelection) {
            var sel = window.getSelection();
            if (sel.rangeCount) {
                var range = sel.getRangeAt(0).cloneRange();
                range.surroundContents(span);
                sel.removeAllRanges();
                sel.addRange(range);
            }
        }
    }

<input type = "button" onclick = "surroundSelection()" value = "Surround">
    <div contenteditable = "true">Hello World</div>