ritical_css_content = str_replace( '\\', '\\\\', $critical_css_content );

		$buffer = preg_replace(
			'#</title>#iU',
			'</title><style id="rocket-critical-css">' . wp_strip_all_tags( $critical_css_content ) . '</style>',
			$buffer,
			1
		);

		return preg_replace( '#</body>#iU', $this->return_remove_cpcss_script() . '</body>', $buffer, 1 );
	}

	/**
	 * Returns JS script to remove the critical css style from frontend.
	 *
	 * @since 3.6
	 *
	 * @return string
	 */
	protected function return_remove_cpcss_script() {
		$filename = rocket_get_constant( 'SCRIPT_DEBUG' ) ? 'cpcss-removal.js' : 'cpcss-removal.min.js';
		$script   = rocket_get_constant( 'WP_ROCKET_PATH' ) . "assets/js/{$filename}";

		if ( ! is_readable( $script ) ) {
			return '';
		}

		return sprintf(
			'<script>%s</script>',
			$this->filesystem->get_contents( $script )
		);
	}

	/**
	 * Adds wprRemoveCPCSS to excluded inline JS array.
	 *
	 * @since 3.6
	 *
	 * @param array $excluded_inline Array of inline JS excluded from being combined.
	 *
	 * @return array
	 */
	public function exclude_inline_js( array $excluded_inline ) {
		$excluded_inline[] = 'wprRemoveCPCSS';

		return $excluded_inline;
	}

	/**
	 * Defer loading of CSS files.
	 *
	 * @since  2.10
	 *
	 * @param string $buffer HTML code.
	 *
	 * @return string Updated HTML code
	 */
	public function async_css( $buffer ) {
		if ( ! $this->should_async_css() ) {
			return $buffer;
		}

		if (
			empty( $this->critical_css->get_current_page_critical_css() )
			&&
			empty( $this->options->get( 'critical_css', '' ) )
		) {
			return $buffer;
		}

		$excluded_css = array_flip( $this->critical_css->get_exclude_async_css() );

		/**
		 * Filters the pattern used to get all stylesheets in the HTML.
		 *
		 * @since  2.10
		 *
		 * @param string $css_pattern Regex pattern to get all stylesheets in the HTML.
		 */
		$css_pattern = apply_filters(
			'rocket_async_css_regex_pattern',
			'/(?=<link[^>]*\s(rel\s*=\s*[\'"]stylesheet["\']))<link[^>]*\shref\s*=\s*[\'"]([^\'"]+)[\'"](.*)>/iU'
		);

		// Remove comments from the buffer.
		$clean_buffer = $this->hide_comments( $buffer );
		$clean_buffer = $this->hide_noscripts( $clean_buffer );

		// Get all css files with this regex.
		preg_match_all( $css_pattern, $clean_buffer, $tags_match );
		if ( ! isset( $tags_match[0] ) ) {
			return $buffer;
		}

		$noscripts = '<noscript>';

		foreach ( $tags_match[0] as $i => $tag ) {
			// Strip query args.
			$path = wp_parse_url( $tags_match[2][ $i ], PHP_URL_PATH );

			// Check if this file should be deferred.
			if ( isset( $excluded_css[ $path ] ) ) {
				continue;
			}

			if ( preg_match( '/media\s*=\s*[\'"]print[\'"]/i', $tags_match[0][ $i ] ) ) {
				continue;
			}

			$preload = str_replace( 'stylesheet', 'preload', $tags_match[1][ $i ] );
			$onload  = preg_replace( '~' . preg_quote( $tags_match[3][ $i ], '~' ) . '~iU', ' data-rocket-async="style" as="style" onload="" onerror="this.removeAttribute(\'data-rocket-async\')" ' . $tags_match[3][ $i ] . '>', $tags_match[3][ $i ] );
			$tag     = str_replace( $tags_match[3][ $i ] . '>', $onload, $tag );
			$tag     = str_replace( $tags_match[1][ $i ], $preload, $tag );
			$tag     = str_replace( 'onload=""', 'onload="this.onload=null;this.rel=\'stylesheet\'"', $tag );
			$tag     = preg_replace( '/(id\s*=\s*[\"\'](?:[^\"\']*)*[\"\'])/i', '', $tag );
			$buffer  = str_replace( $tags_match[0][ $i ], $tag, $buffer );

			$noscripts .= $tags_match[0][ $i ];
		}

		$noscripts .= '</noscript>';

		return str_replace( '</body>', $noscripts . '</body>', $buffer );
	}

	/**
	 * Regenerates the CPCSS when switching theme if the option is active.
	 *
	 * @since 3.3
	 */
	public function maybe_regenerate_cpcss() {
		if ( ! $this->options->get( 'async_css' ) ) {
			return;
		}

		if ( ! $this->is_mobile_cpcss_active() ) {
			$this->critical_css->process_handler( 'default', 'all' );
			return;
		}

		$this->critical_css->process_handler( 'all' );
	}

	/**
	 * Checks if mobile CPCSS is active.
	 *
	 * @since 3.6
	 *
	 * @return boolean CPCSS active or not.
	 */
	private function is_mobile_cpcss_active() {
		return (
			$this->options->get( 'async_css', 0 )
			&&
			$this->options->get( 'cache_mobile', 0 )
			&&
			$this->options->get( 'do_caching_mobile_files', 0 )
		)
		&&
		$this->options->get( 'async_css_mobile', 0 );
	}

	/**
	 * Checks if we should async CSS
	 *
	 * @since 3.6.2.1
	 *
	 * @return boolean
	 */
	private function should_async_css() {
		if ( rocket_get_constant( 'DONOTROCKETOPTIMIZE' ) ) {
			return false;
		}

		if ( ! $this->options->get( 'async_css', 0 ) ) {
			return false;
		}

		return ! is_rocket_post_excluded_option( 'async_css' );
	}

	/**
	 * Stops the critical CSS generation.
	 *
	 * @since 3.10
	 *
	 * @return void
	 */
	public function stop_critical_css_generation() {

		$this->critical_css->stop_generation();
		delete_transient( 'rocket_critical_css_generation_process_running' );
		delete_transient( 'rocket_critical_css_generation_process_complete' );
	}

	/**
	 * Display a notice to pass from CPCSS to RUCSS.
	 *
	 * @return void
	 */
	public function switch_to_rucss_notice() {
		$boxes = get_user_meta( get_current_user_id(), 'rocket_boxes', true );

		if ( in_array( __FUNCTION__, (array) $boxes, true ) ) {
			return;
		}

		if ( ! $this->options->get( 'async_css', 0 ) ) {
			return;
		}

		if ( $this->user->is_license_expired() ) {
			return;
		}

		$screen = get_current_screen();

		if ( isset( $screen->id ) && 'settings_page_wprocket' !== $screen->id ) {
			return;
		}

		/**
		 * Filters the status of the RUCSS option.
		 *
		 * @param array $should_disable will return array with disable status and text.
		 */
		$rucss_status = apply_filters(
			'rocket_disable_rucss_setting',
			[
				'disable' => false,
				'text'    => '',
			]
		);

		if ( is_array( $rucss_status ) && key_exists( 'disable', $rucss_status ) && $rucss_status['disable'] ) {
			return;
		}

		rocket_notice_html(
			[
				'status'                 => 'wpr-warning',
				'dismissible'            => '',
				'dismiss_button'         => __FUNCTION__,
				'message'                => sprintf(
					// translators: %1$ = opening bold tag, %2$ = closing bold tag.
					__( 'We highly recommend the %1$supdated Remove Unused CSS%2$s for a better CSS optimization. Load CSS Asynchronously is always available as a back-up.', 'rocket' ),
					'<b>',
							'</b>'
				),
				'action'                 => 'switch_to_rucss',
				'dismiss_button_message' => __( 'Stay with the old option', 'rocket' ),
			]
		);
	}

	/**
	 * Switch to RUCSS.
	 *
	 * @return void
	 */
	public function switch_to_rucss() {
		check_admin_referer( 'rucss_switch' );

		if ( ! current_user_can( 'rocket_manage_options' ) ) {
			wp_safe_redirect( wp_get_referer() );
			rocket_get_constant( 'WP_ROCKET_IS_TESTING', false ) ? wp_die() : exit;
			return; // phpcs:ignore Squiz.PHP.NonExecutableCode.Unreachable
		}

		$this->options->set( 'async_css', false );
		$this->options->set( 'remove_unused_css', true );
		$this->options_api->set( 'settings', $this->options->get_options() );

		rocket_dismiss_box( 'switch_to_rucss_notice' );

		wp_safe_redirect( wp_get_referer() );
		rocket_get_constant( 'WP_ROCKET_IS_TESTING', false ) ? wp_die() : exit;
	}
}