How can I highlight the text of the DOM Range object?

I would suggest using document‘s or the TextRange‘s execCommand method, which is built for just such a purpose, but is usually used in editable documents. Here’s the answer I gave to a similar question:

The following should do what you want. In non-IE browsers it turns on designMode, applies a background colour and then switches designMode off again.

UPDATE

Fixed to work in IE 9.

UPDATE 12 September 2013

Here’s a link detailing a method for removing highlights created by this method:

https://stackoverflow.com/a/8106283/96100

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;
    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);
    }
}