We need AJAX CROSS DOMAIN

[nvcnvn]

For now, many language (like php) allow You use there libary to send POST or GET data to any domain (like cURLs in PHP).
Even some js lia\bary allow to do this with some more external script.

So we don't have to keep a powerfull function in one singel domain.
Just more powerfull when javascript in HTML5 can send the data all around the world!

[mskinner]

Javascript keyloggers would become an identity thief's best friend.

[zcorpan]

XHR level 2 allows cross-origin requests if the server allows it (using CORS-related HTTP headers).

Keylogging isn't an issue since you can already send data to a remote origin with e.g. the <img> element, so that's not the reason XHR is same-origin by default.