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)
Returns the cleaned CSS

        :param stylesheet: The Stylesheet object to parse
        :type stylesheet: tinycss.css21.Stylesheet