public function filter_update_custom_css_data( $data, $args ) {
    $this->assertIsArray( $data );
    $this->assertSameSets( array( 'css', 'preprocessed' ), array_keys( $data ) );
    $this->assertSame( '', $data['preprocessed'] );
    $this->assertIsArray( $args );
    $this->assertSameSets( array( 'css', 'preprocessed', 'stylesheet' ), array_keys( $args ) );
    $this->assertSame( $args['css'], $data['css'] );
    $this->assertSame( $args['preprocessed'], $data['preprocessed'] );

    $data['css']         .= '/* filtered post_content */';
    $data['preprocessed'] = '/* filtered post_content_filtered */';
    $data['post_title']   = 'Ignored';
    return $data;
  }