The IE team write about some of the latest rounds of enhancements on improving AJAX updates in Internet Explorer Beta 2 focus on maintaining cross-browser compatibility and the feature sets that developers have thought would be the most useful.
XDomainRequest (XDR): This is an object built from the ground up to make client-side cross-domain calls secure and easy. To reduce the chances of inadvertent cross domain access, this object requires an explicit acknowledgement for allowing cross domain calls from the client script and the server. Additionally, it reduces the need for sites having to resort to the dangerous practice of merge scripting from third parties directly into the mashup page. This practice is dangerous because it provides third parties full access to the DOM. All this comes with the added benefit of improved client-side performance and lower server maintenance costs thanks to the absence of a need for server farms for proxying.[…]