docs: def _clean_css(self):
          """
          Returns the cleaned CSS
  
          :param stylesheet: The Stylesheet object to parse
          :type stylesheet: tinycss.css21.Stylesheet
          """
          # Init the cleaned CSS rules and contents string
          css_rules = []
  
          # For every rule in the CSS
          for rule in self.stylesheet.rules:
  
              try:
                  # Clean the CSS rule
                  cleaned_rule = self._clean_rule(rule)
  
                  # Append the rule to matched CSS rules
                  if cleaned_rule is not None:
                      css_rules.append(cleaned_rule)
  
              except:
                  # On error, assume the rule matched the tree
                  css_rules.append(rule)
  
          return self._build_css(css_rules)