Change CSS of selected text using Javascript

I'm trying to make a JavaScript bookmarklet that will act as a highlighter, changing the background of selected text on a webpage to yellow when the bookmarklet is pressed.

I'm using the following code to get the selected text, and it works fine, returning the correct string

function getSelText() {
    var SelText = '';
    if (window.getSelection) {
        SelText = window.getSelection();
    } else if (document.getSelection) {
        SelText = document.getSelection();
    } else if (document.selection) {
        SelText = document.selection.createRange().text;
    }
    return SelText;
}

However, when I created a similar function to change the CSS of the selected text using jQuery, it isn't working:

function highlightSelText() {
    var SelText;
    if (window.getSelection) {
        SelText = window.getSelection();
    } else if (document.getSelection) {
        SelText = document.getSelection();
    } else if (document.selection) {
        SelText = document.selection.createRange().text;
    }
    $(SelText).css({'background-color' : 'yellow', 'font-weight' : 'bolder'});
}

Any ideas?

The easiest way to do this is to use execCommand(), which has a command to change the background colour in all modern browsers.

The following should do what you want on any selection, including ones spanning multiple elements. In non-IE browsers it turns on designMode, applies a background colour and then switches designMode off again.

UPDATE

Fixed in IE 9.

function makeEditableAndHighlight(colour) {
    var range, sel = window.getSelection();
    if (sel.rangeCount && sel.getRangeAt) {
        range = sel.getRangeAt(0);
    }
    document.designMode = "on";
    if (range) {
        sel.removeAllRanges();
        sel.addRange(range);
    }
    // Use HiliteColor since some browsers apply BackColor to the whole block
    if (!document.execCommand("HiliteColor", false, colour)) {
        document.execCommand("BackColor", false, colour);
    }
    document.designMode = "off";
}

function highlight(colour) {
    var range, sel;
    if (window.getSelection) {
        // IE9 and non-IE
        try {
            if (!document.execCommand("BackColor", false, colour)) {
                makeEditableAndHighlight(colour);
            }
        } catch (ex) {
            makeEditableAndHighlight(colour)
        }
    } else if (document.selection && document.selection.createRange) {
        // IE <= 8 case
        range = document.selection.createRange();
        range.execCommand("BackColor", false, colour);
    }
}