'Extract the unique set of class names used in the content'

@newrelic.agent.function_trace()
def css_classnames(self):

    classnames = set()
    for element in pq(self.document.rendered_html).find('*'):
        css_classes = element.attrib.get('class')
        if css_classes:
            classnames.update(css_classes.split('    '))
    return list(classnames)
