The fourth platform of IE10 simplifies building cross-site scenarios that work consistently across browsers by supporting Cross-Origin Resource Sharing (CORS) for XMLHttpRequest (XHR). CORS for XHR makes sharing data across sites simple and flexible. In the most basic scenario CORS enables creating data sources accessible from any site, and with a few small tweaks you can choose to constrain allowed sites, support data modification, and even allow authentication. Most importantly CORS keeps existing sites secure by requiring server participation. - Andrew Mason