Update the XSLTProcessor tentative IDL (#28535)

diff --git a/interfaces/xslt.tentative.idl b/interfaces/xslt.tentative.idl
index e97d8b0..8e07d86 100644
--- a/interfaces/xslt.tentative.idl
+++ b/interfaces/xslt.tentative.idl
@@ -1,16 +1,17 @@
-// Content was manually copied July 12 2019
+// Content was manually copied April 16 2021
 // Source: https://wiki.whatwg.org/wiki/DOM_XSLTProcessor
 // There is no standard for this API. Only the link above, and notes in
 // the HTML5 standard (https://html.spec.whatwg.org/multipage/scripting.html#scriptTagXSLT)
 
-[Exposed=Window, Constructor]
+[Exposed=Window]
 interface XSLTProcessor {
-  void importStylesheet(Node style);
+  constructor();
+  undefined importStylesheet(Node style);
   [CEReactions] DocumentFragment transformToFragment(Node source, Document output);
   [CEReactions] Document transformToDocument(Node source);
-  void setParameter([TreatNullAs=EmptyString] DOMString namespaceURI, DOMString localName, any value);
-  any getParameter([TreatNullAs=EmptyString] DOMString namespaceURI, DOMString localName);
-  void removeParameter([TreatNullAs=EmptyString] DOMString namespaceURI, DOMString localName);
-  void clearParameters();
-  void reset();
+  undefined setParameter([LegacyNullToEmptyString] DOMString namespaceURI, DOMString localName, any value);
+  any getParameter([LegacyNullToEmptyString] DOMString namespaceURI, DOMString localName);
+  undefined removeParameter([LegacyNullToEmptyString] DOMString namespaceURI, DOMString localName);
+  undefined clearParameters();
+  undefined reset();
 };