^引用
... but when you try to change document.domain back to the original value you get an error. From Firefox:

Error: [Exception... "Illegal document.domain value"
code: “1009″
nsresult: “0×805303f1 (NS_ERROR_DOM_BAD_DOCUMENT_DOMAIN)”
location: “http://www.fettig.net/playground/ajax-subdomain/test3-iframe.html
Line: 13″]

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. ......