'Get the appropriate \' class="..."\' string (note the leading
space), if any, for the given tag.'

def _html_class_str_from_tag(self, tag):

    if ('html-classes' not in self.extras):
        return ''
    try:
        html_classes_from_tag = self.extras['html-classes']
    except TypeError:
        return ''
    else:
        if (tag in html_classes_from_tag):
            return ('    class="%s"' % html_classes_from_tag[tag])
    return ''
