When did it become good to use UTF-8?

Question

When did people start using UTF-8 in file encodings and HTTP Content-Type headers? Since all web servers, OSs, text editors, and browsers support it today, when did it become “compatible” between them?

Answer 1

This picture may be useful:

Edit: I found a later version :

Answer 2

UTF-8 has always been backward compatible with ASCII .

In principle, it was normal to use UTF-8, since it was normal to use ASCII (which is quite a long time).