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.
Precedence: HTTP vs HTML5 meta
The symbol should describe the two lines.
Assertion: The HTTP header has a higher precedence than an HTML5 meta encoding declaration.
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 HTTP header declares ISO 8859-15; the HTML5 meta charset, ISO 8859-1.
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 HTML5 meta charset element will cause the pages to be non-valid.