Eu escrevi um artigo sobre isso há algum tempo. Extrair:
Safari decided to disable Cherokee,
Cyrillic, and Greek scripts by
default. These three scripts all have
many Latin lookalike characters. This
is well and good if you have a
majority English-speaking audience,
but not ideal for the rest of the
world. And it certainly doesn't solve
all the problems. Latin characters
with unusual diacritics will still
work, and some fonts leave out some of
these diacritics, so that l and
l-cedila may well look identical.
Opera, and later Firefox, took the
position that this was a register
issue: Internet registers should not
allocate domains such as
www.xn--pypal-4ve.com in the first
place. These browser manufacturers
maintain a whitelist of 'well behaved'
registrars, who don't allow spoofing
domains in their registry. For
example, it would be impossible to
register www.xn--pypal-4ve.info,
because the .info registrar wouldn't
allow it. Opera's whitelist of TLDs
is built into the browser. Firefox's
list is on display. It's worth noting
that the most popular registrar, .com,
allocates domains strictly on a
first-come-first-served basis, with no
checks at all. Therefore, IDNs in .com
will not work in Opera or Firefox.
Google's new Chrome browser has IDNAs
turned off by default.
Internet Explorer is, of course, more
integrated with the operating system
than most browsers are, so it checks
what language supports are configured
within Windows. If the script in the
IDN is part of the user's configured
accept language, the name will display
as an IDNA. Otherwise, the punycode
will be shown. However, when scripts
are mixed (Cyrillic and Latin in the
same label, for example), the punycode
will be shown, even if Cyrillic (in
our example) is usually accepted. Some
scripts which look nothing like Latin
are allowed to mix, as these don't
present a threat.