private function _renderItemsTogetherAsGroup ($itemsToRender = [], $minify = FALSE) {
		// complete tmp filename by source filenames and source files modification times
		$filesGroupInfo = [];
		foreach ($itemsToRender as $item) {
			if (self::$fileChecking) {
				$fullPath = $this->getAppRoot() . $item->path;
				if (!file_exists($fullPath)) {
					$this->exception("File not found in CSS view rendering process ('$fullPath').");
				}
				$filesGroupInfo[] = $item->path . '?_' . self::getFileImprint($fullPath);
			} else {
				$filesGroupInfo[] = $item->path;
			}
		}
		$tmpFileFullPath = $this->getTmpFileFullPathByPartFilesInfo($filesGroupInfo, $minify, 'css');

		// check, if the rendered, together completed and minimized file is in tmp cache already
		if (self::$fileRendering) {
			if (!file_exists($tmpFileFullPath)) {
				// load all items and join them together
				$resultContent = '';
				foreach ($itemsToRender as & $item) {
					$srcFileFullPath = $this->getAppRoot() . $item->path;
					if ($item->render) {
						$fileContent = $this->_renderFile($srcFileFullPath);
					} else if ($minify) {
						$fileContent = file_get_contents($srcFileFullPath);
					}
					$fileContent = $this->_convertStylesheetPathsFromRelatives2TmpAbsolutes(
						$fileContent, $item->path
					);
					if ($minify) $fileContent = $this->_minify($fileContent, $item->path);
					$resultContent .= PHP_EOL . "/* " . $item->path . " */" . PHP_EOL . $fileContent . PHP_EOL;
				}
				// save completed tmp file
				$this->saveFileContent($tmpFileFullPath, $resultContent);
				$this->log("Css files group rendered ('$tmpFileFullPath').", 'debug');
			}
		}

		// complete <link> tag with tmp file path in $tmpFileFullPath variable
		$firstItem = array_merge((array) $itemsToRender[0], []);
		$pathToTmp = substr($tmpFileFullPath, strlen($this->getAppRoot()));
		$firstItem['href'] = $this->CssJsFileUrl($pathToTmp);
		return $this->_renderItemSeparated((object) $firstItem);
	}