def replace_css_class(html_string, old_class, new_class):
    """
    Replaces the css class in teh html element string.
    :param html_string: a string of html
    :param old_class: the text segment to replace. It should be a css class designator
    :param new_class: the text to add in place of the old css class 
    :return: a string of html
    """
    return html_string.replace(old_class, new_class)