Test:

[BOM] <!DOCTYPE HTML>
<meta charset="ISO-8859-9">

Expected result: UTF-8

Encoding used by browser is: Script did not run.

Result: Script did not run.

Then, temporarily override the encoding to be Celtic ISO-8859-14 to see if it changes both. It should (i.e. after you change the encoding, it should say FAIL above, with "ïṠṡ" at the top of the page).