This method is called periodically as an event handler for progress events on XDomainRequests, so that code can monitor progress while loading content.
document.load() is a part of an old version of the W3C DOM Level 3 Load & Save module. Can be used with document.async to indicate whether the request is synchronous or asynchronous (the default). As of at least Gecko 1.9, this no longer supports cross-site loading of documents (Use XMLHttpRequest instead).
XMLHttpRequest makes sending HTTP requests very easy. You simply create an instance of the object, open a URL, and send the request. The HTTP status of the result, as well as the result's contents, are available in the request object when the transaction is completed. This page outlines some of the common, and even slightly obscure, use cases for this powerful JavaScript object.