Simplified the isXML function, no need to use recursion.

src/core.js

@@ -291,9 +291,12 @@ jQuery.fn = jQuery.prototype = {
 			jQuery.find( selector, this[0], ret );
 			return ret;
 		} else {
-			return this.pushStack( jQuery.unique(jQuery.map(this, function(elem){
-				return jQuery.find( selector, elem );
-			})), "find", selector );
+			var ret = this.pushStack( "", "find", selector );
+			for ( var i = 0, l = this.length; i < l; i++ ) {
+				jQuery.find( selector, this[i], ret );
+			}
+			//jQuery.find.uniqueSort( ret );
+			return ret;
 		}
 	},
 

src/selector.js

@@ -941,7 +941,7 @@ var contains = document.compareDocumentPosition ?  function(a, b){
 
 var isXML = function(elem){
 	return elem.nodeType === 9 && elem.documentElement.nodeName !== "HTML" ||
-		!!elem.ownerDocument && isXML( elem.ownerDocument );
+		!!elem.ownerDocument && elem.ownerDocument.documentElement.nodeName !== "HTML";
 };
 
 var posProcess = function(selector, context){