A technology which enables clients to retrieve and submit XML data directly, all in the background. This enables web pages to update their content without needing to reload the entire page. Conversion of retrieved XML data into renderable HTML content relies on the client-side Document Object Model (DOM) to read the XML document node tree and compose HTML elements that the user sees.
Microsoft first implemented the XMLHttpRequest object in Internet Explorer 5 for Windows as an ActiveX object. Engineers on the Mozilla project implemented a compatible native version for Mozilla 1.0 (and Netscape 7). Apple has done the same starting with Safari 1.2, and Opera starting with V7.6.
While offering great opportunities for web developers, XMLHttpRequest is still having some issues that prevent it from being used to the fullest extent: