Всё работает,но валидатор не пропускает. Сделал через JSOUP.

public class Solution {
    public static void main(String[] args) {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            String fileName = reader.readLine();
            String resultLine = "";
            FileReader fileReader = new FileReader(fileName);
            BufferedReader bufferedReader = new BufferedReader(fileReader);
            int symbol = bufferedReader.read();
            while (symbol != -1) {
                resultLine = resultLine.concat(String.valueOf((char) symbol));
                symbol = bufferedReader.read();
            }
            bufferedReader.close();
            String tag = args[0];
            Document doc = Jsoup.parse(resultLine);
            Elements inputElements = doc.select(tag);
            for (Element inputElement : inputElements) {
                System.out.println(inputElement);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}