 may be synchronous or asynchronous.</p>
<p>All event handlers in this class except onreadystatechange receive an Object containing the following fields:
"lengthComputable", a boolean indicating whether the total size of the download could be calculated before completion,
"loaded", the number of bytes downloaded so far, and "total", the total size of the HTTP response body if known, otherwise
0. The "this" keyword may be used to refer to the XMLHttpRequest instance inside its event handler callbacks.</p></d