Ensure that you have fonts that can represent all characters in ISO 8859-15. Ensure that the default encoding for the browser you are testing is not set to ISO 8859-15.
HTTP vs UTF-8 BOM
Test passes if the lines below look the same.
Assertion: If the HTTP header of a page is not set to UTF-8, a UTF-8 BOM will not cause a file to be treated as UTF-8.
No encoding information is declared in the document.
The text contains the following sequence of bytes: 0xC3 0xBD 0xC3 0xA4 0xC3 0xA8. The characters serve to distinguish ISO 8859-15 from ISO 8859-1 and UTF-8.
The graphic on the lower line shows what you would expect to see if the page is encoded as ISO 8859-15. The large symbol indicates whether the characters on the two lines should be the same (equal sign) or different (not-equal sign).
The characters should appear at the top of the page. These represent the bytes that make up the UTF-8 when encountered in the ISO 8859-15 encoding. This will cause syntax errors for validation of the html, and will stop the display of the xml.