If you see query marks in place of accented characters in messages, it's probably not because your browser is too limited but because the poster pasted in raw text, without replacing the accented characters with HTML escapes. For instance, an accented u should be entered as "ú", which will then appear to everyone else as "ú". (You see it correctly here, don't you?) The Scots Gaelic equivalent (ù) is entered as "ù".

If posters mark the Preview box when entering text cut-n-pasted from other source, they can catch most of these problems. Note that the characters that many word processors use for quotes and dashes are also in the "extended" range, and may show up as query marks (unrecognized characters) when pasted into messages. They need to be replaced with ASCII equivalents or HTML escapes.

Celtic (vs. Romanized) text with dotted and "long" consonants are another story. Whether they appear correctly depends on what font you have selected as your default. Many fonts don't include these characters at all; others include the dotted consonants, but not the long ones or 7-ampersand (agus). Fortunately, most folks here enter Romanized text, with trailing h's indicating dotted consonants.