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.
HTML5 meta, then meta Content-Type
Test passes if the lines below look the same.
Assertion: An HTML5 meta encoding declaration has a higher precedence than a following meta Content-Type 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 HTML5 meta charset declares ISO 8859-15; the following meta Content-Type, 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).
This page does not validate when not served as HTML5, because of the meta charset markup.