def sort_urls(urls):
    """
    Order URLs by extension.
    This function accepts a list of URLs and orders them by their extension.
    CSS files are sorted to the start of the list, then JS, then everything
    else.
    """
    order = {'css': 0, 'js': 1}
    urls.sort(key=lambda x: order.get(x.rsplit('.')[-1].lower(), 2))
    return urls
