Currently, Opera doesn't support ibm864 at all. And Gecko converts 0x25 to U+066A (ARABIC PERCENT SIGN). I don't think it is practical to use the encoding lacking U+0025 (PERCENT SIGN) on the web.
Gecko will stop exposing the ibm864 support as soon as the spec has been updated.
Removed in http://encoding.spec.whatwg.org/ as per https://github.com/whatwg/encoding/commit/e96c989a55bb76b2f32f05ea8b86476cd219337a