XMLHttpRequest responseXML in IE10 Release Preview

IE10 in Windows 8 Release Preview updates the responseXML from an XMLHttpRequest to return a native XML document by default. This change applies to IE10’s Standards and Quirks document modes, making them interoperable with other modern browsers and consistent with a “same markup” approach. Compatibility document modes 5, 7, 8, and 9 are unchanged.
This change may impact sites that were expecting responseXML to contain an MSXML document and depended on MSXML-specific functionality such as selectNodes. In these cases, you may request that IE10 return an MSXML by setting the responseType member of your XMLHttpRequest object to 'msxml-document'. If your code does not depend on MSXML-specific functionality, IE10’s native XML document should work for you.
Native XML support in IE9 brought DOM parity to XML and HTML and enabled XML fragments to be inserted and rendered directly within a page (even in HTML). IE9 also simplified converting between XML and DOM with the addition of DOMParser and XMLSerializer. IE10 completes this transition by updating responseXML to return a native XML document.
Like IE9, IE10 previews before the Windows 8 Release Preview returned an MSXML document for responseXML. As a result, retrieving a native document required the additional step of passing responseText to DOMParser.

var xhr = new XMLHttpRequest();

//...

var parser = new DOMParser();

var doc = parser.parseFromString(xhr.responseText, 'text/xml');

// 'doc' contains a native document in both IE9 and IE10

In the Windows 8 Release Preview, IE10 eliminates the need for an additional DOMParser step by returning a native document directly via responseXML. Existing code using DOMParser will continue to work in IE10.

var xhr = new XMLHttpRequest();

//...

var doc = xhr.responseXML;

// 'doc' contains a native document in IE10’s Standards and Quirks document modes

// it contains an MSHTML document in IE9 and in IE10’s compatibility document modes

This simplification also applies to the new response property when responseType is set to 'document'.

var xhr = new XMLHttpRequest();

xhr.open(method, url, true);

xhr.responseType = 'document';

//...

var doc = xhr.response;

// 'doc' contains a native document in IE10’s Standards and Quirks document modes

IE10 additionally includes a mechanism to opt-in to retrieving an MSXML document. This can be useful if you still need some MSXML-specific functionality (such as selectNodes) or simply need some extra time to migrate. To do this, set the responseType of your XMLHttpRequest object to 'msxml-document'.

var xhr = new XMLHttpRequest();

xhr.open(method, url, true);

try { xhr.responseType = 'msxml-document'; } catch(e){}

//...

var doc = xhr.responseXML;

// 'doc' now contains an MSXML document in IE10’s Standards and Quirks document modes

In theory the assignment should be ignored by other browsers, but in practice some do throw an exception. You can defend against this using a try/catch statement, as in the above example.
—Tony Ross, Program Manager, Internet Explorer