code: def to_css_class(s):
      
      if css_class_re.match(s):
          return s
      md5_suffix = hashlib.md5(s.encode('utf8')).hexdigest()[:6]
      # Strip leading _, -
      s = s.lstrip('_').lstrip('-')
      # Replace any whitespace with hyphens
      s = '-'.join(s.split())
      # Remove any remaining invalid characters
      s = css_invalid_chars_re.sub('', s)
      # Attach the md5 suffix
      bits = [b for b in (s, md5_suffix) if b]
      return '-'.join(bits)
docs: Given a string (e.g. a table name) returns a valid unique CSS class.
      For simple cases, just returns the string again. If the string is not a
      valid CSS class (we disallow - and _ prefixes even though they are valid
      as they may be confused with browser prefixes) we strip invalid characters
      and add a 6 char md5 sum suffix, to make sure two tables with identical
      names after stripping characters don't end up with the same CSS class.