When did it become OK to use UTF-8?

When did people start using UTF-8 in files encoding and HTTP Content-Type headers? Since all web servers, OSes, text editors and browsers support it开发者_运维知识库 today, when did it become "compatible" between these?

---

this picture might be useful:

Edit: I found a later version:

---

UTF-8 has always been backwards compatible with ASCII.

So basically, it's been OK to use UTF-8 since it's been OK to use ASCII (which is quite a long time).