Enable only remove operation for Document::update_idmap().

src/components/script/dom/document.rs

@@ -406,24 +406,34 @@ impl Document {
 
     pub fn update_idmap(&mut self,
                         abstract_self: AbstractNode,
-                        new_id: DOMString,
+                        new_id: Option<DOMString>,
                         old_id: Option<DOMString>) {
-        // remove old ids if the old ones are not same as the new one.
+        // remove old ids:
+        // * if the old ones are not same as the new one,
+        // * OR if the new one is none.
         match old_id {
-            Some(ref old_id) if new_id != *old_id => {
+            Some(ref old_id) if new_id.is_none() ||
+                                (*new_id.get_ref() != *old_id) => {
                 self.idmap.remove(old_id);
             }
             _ => ()
         }
 
-        // TODO: support the case if multiple elements which haves same id are in the same document.
+        match new_id {
-        self.idmap.mangle(new_id, abstract_self, |_, new_node: AbstractNode| -> AbstractNode {
+            Some(new_id) => {
+                // TODO: support the case if multiple elements
+                // which haves same id are in the same document.
+                self.idmap.mangle(new_id, abstract_self,
+                                  |_, new_node: AbstractNode| -> AbstractNode {
                                       new_node
                                   },
                                   |_, old_node: &mut AbstractNode, new_node: AbstractNode| {
                                       *old_node = new_node;
                                   });
             }
+            None => ()
+        }
+    }
 }
 
 #[inline(always)]

src/components/script/dom/element.rs

@@ -221,7 +221,7 @@ impl Element {
             "id" => {
                 let doc = self.node.owner_doc();
                 let doc = doc.mut_document();
-                doc.update_idmap(abstract_self, value.clone(), old_value);
+                doc.update_idmap(abstract_self, Some(value.clone()), old_value);
             }
             _ => ()
         }

src/test/html/content/test_document_getElementById.html

@@ -59,6 +59,11 @@
 
       let old = document.getElementById(TEST_ID);
       is(old, null, "test3-1, the method shouldn't get the element by the old id.");
+
+      // remove id.
+      test.removeAttribute("id");
+      let e2 = document.getElementById(UPDATED_ID);
+      is(e2, null, "test3-2, the method should return null when the passed id is none in document.");
     }
 
     // TODO: