The W3C Web API Working Group has posted the third public working draft of Progress Events 1.0. This "defines events which can be used to monitor a process and provide feedback to a user, particularly for network-based events." Here's the IDL:
interface ProgressEvent : events::Event { readonly attribute boolean lengthComputable; readonly attribute unsigned long loaded; readonly attribute unsigned long total; void initProgressEvent(in DOMString typeArg, in boolean canBubbleArg, in boolean cancelableArg, in boolean lengthComputableArg, in unsigned long loadedArg, in unsigned long totalArg, void initProgressEventNS(in DOMString namespaceURI, in DOMString typeArg, in boolean canBubbleArg, in boolean cancelableArg, in boolean lengthComputableArg, in unsigned long loadedArg, in unsigned long totalArg, };