The letter ə becomes a question mark on the page

If the browser does not know the file's encoding, the letter ə becomes ? or a strange mark. lang="az" does not fix the letter. The line that does is <meta charset="UTF-8">. The file itself must be saved as UTF-8.

Do not change everything at once. Check the cause in order.

• One of the first lines inside <head> should be <meta charset="UTF-8">.
• The bottom corner of VS Code should say UTF-8. In Notepad, Unicode means UTF-16. Do not pick that.
• lang="az" names the language of the page. It does not store the letter.
• If the file was saved as ANSI or cp1254, ə is already gone. Switch to UTF-8 and type the letter again.
• If the server sends another encoding in Content-Type, that comes before the meta line. On a static page, meta is enough.

An Excel CSV is a different file. There utf-8 is not enough, and utf-8-sig is needed. HTML does not use sig.

<!DOCTYPE html>
<html lang="az">
<head>
  <meta charset="UTF-8">
  <title>Səhifə</title>
</head>
<body>
  <p>Azərbaycan</p>
</body>
</html>