【Tip】document.domain

Mozilla and Opera are more strict about the value of
document.domain – it can only be set to the current value or a
higher-level domain. For example, if the host is aaa.bbb.example.com, I
could set document.domain to bbb.example.com. At that point I could
change it again, to example.com, but I couldn’t change it back to
aaa.bbb.example.com. Once you move to a higher-level domain, you’re
stuck there.