Mozilla

Tested on:
1) Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.16) Gecko/20080702 SeaMonkey/1.1.11 (via squid/2.5.STABLE7)

- 8-bit text in menus is converted to ISO-8859-1. UTF-8 in menus is treated as 8-bit text.

2) Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.0.14eol) Gecko/20070505 Iceape/1.0.9 (Debian-1.0.13~pre080323b-0etch3)

- UTF-8 symbols (tested with cyrillic range) displayed as quetion marks
- KOI-8 is not displayed at all
- The same with Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.12) Gecko/2008012900 Iceweasel/2.0.0.12 (Debian-2.0.0.12-0etch1)
 
UMN gopher and Lynx

- KOI-8 symbols are OK with KOI-8 locale and correct fonts in xterm
- UTF-8 symbols are treated as 8-bit in xterm and corrupted in uxterm
(locale is irrelevant)
- Looks like Lynx's charset options do not affect gopher appearance

OverbiteFF (latest version)

- With a 8-bit encoding set as browser's default encoding, 8-bit text in that encoding is displayed correctly out of the box. Need to explicitly set encoding for utf-8 text in that case. Didn't check what happens with 8-bit encoding if default is utf-8.

(Corruption of UTF-8 in both clients is the same and  can be related to some OS issues so
it needs more testing)