Adding xhtml+xml capabitlity to ajax load()

[Max_B]

Currently, the load() method uses the responseText from the ajax request and innerHTML. If the server send application/xhtml+xml content-type header the responseXML is returned by the ajax/get/post method but is not inserted into the DOM.

One needs to manually add it in the callback with a code like:

function (data, status) {
	if (data.documentElement) targetnode.appendChild(document.importNode(data.documentElement, true));
	else targetnode.append(data);
}

[flesler]

document.importNode doesn't exist on IE.

[addyosmani]

Duplicate of #4958.