For XML output, "hex" or "decimal" indicate whether numeric character references should be in hexadecimal or decimal.
For HTML output, two strings separated by a semicolon. The first string defines how non-ASCII characters within the character encoding will be represented, the values being "native", "entity", "decimal", or "hex". The second string defines how characters outside the encoding will be represented, the values being "entity", "decimal", or "hex".