According to w3c: > The error flag indicates some type of network error or request abortion and in their spec for "network errors" at https://www.w3.org/TR/20

According to w3c: > The error flag indicates some type of network error or request abortion and in their spec for "network errors" at https://www.w3.org/TR/2012/WD-XMLHttpRequest-20120117/#request-error https://www.w3.org/TR/2012/WD-XMLHttpRequest-20120117/#reque... unless you're using it in synchronous mode, it just sets the error flag. There's an onerror callback, but it doesn't appear to get any information about what went wrong than the WHATWG version.