diff --git a/src/components/script/dom/htmlcollection.rs b/src/components/script/dom/htmlcollection.rs
index 77d36e07005..71f4f90a568 100644
--- a/src/components/script/dom/htmlcollection.rs
+++ b/src/components/script/dom/htmlcollection.rs
@@ -12,24 +12,40 @@ use dom::window::Window;
 use servo_util::namespace::Namespace;
 use servo_util::str::DOMString;
 
+use serialize::{Encoder, Encodable};
+
+pub trait CollectionFilter {
+    fn filter(&self, elem: &JS<Element>, root: &JS<Node>) -> bool;
+}
+
+impl<S: Encoder> Encodable<S> for ~CollectionFilter {
+    fn encode(&self, _s: &mut S) {}
+}
+
+#[deriving(Encodable)]
+pub enum CollectionTypeId {
+    Static(~[JS<Element>]),
+    Live(JS<Node>, ~CollectionFilter)
+}
+
 #[deriving(Encodable)]
 pub struct HTMLCollection {
-    elements: ~[JS<Element>],
+    collection: CollectionTypeId,
     reflector_: Reflector,
     window: JS<Window>,
 }
 
 impl HTMLCollection {
-    pub fn new_inherited(window: JS<Window>, elements: ~[JS<Element>]) -> HTMLCollection {
+    pub fn new_inherited(window: JS<Window>, collection: CollectionTypeId) -> HTMLCollection {
         HTMLCollection {
-            elements: elements,
+            collection: collection,
             reflector_: Reflector::new(),
             window: window,
         }
     }
 
-    pub fn new(window: &JS<Window>, elements: ~[JS<Element>]) -> JS<HTMLCollection> {
-        reflect_dom_object(~HTMLCollection::new_inherited(window.clone(), elements),
+    pub fn new(window: &JS<Window>, collection: CollectionTypeId) -> JS<HTMLCollection> {
+        reflect_dom_object(~HTMLCollection::new_inherited(window.clone(), collection),
                            window, HTMLCollectionBinding::Wrap)
     }
 }
@@ -45,7 +61,7 @@ impl HTMLCollection {
                 }
             }
         }
-        HTMLCollection::new(window, elements)
+        HTMLCollection::new(window, Static(elements))
     }
 
     pub fn by_tag_name(window: &JS<Window>, root: &JS<Node>, tag_name: DOMString) -> JS<HTMLCollection> {
@@ -66,15 +82,26 @@ impl HTMLCollection {
 impl HTMLCollection {
     // http://dom.spec.whatwg.org/#dom-htmlcollection-length
     pub fn Length(&self) -> u32 {
-        self.elements.len() as u32
+        match self.collection {
+            Static(ref elems) => elems.len() as u32,
+            Live(ref root, ref filter) => root.traverse_preorder()
+                .count(|child| {
+                    let elem: Option<JS<Element>> = ElementCast::to(&child);
+                    elem.map_or(false, |elem| filter.filter(&elem, root))
+                }) as u32
+        }
     }
 
     // http://dom.spec.whatwg.org/#dom-htmlcollection-item
     pub fn Item(&self, index: u32) -> Option<JS<Element>> {
-        if index < self.Length() {
-            Some(self.elements[index].clone())
-        } else {
-            None
+        match self.collection {
+            Static(ref elems) => elems
+                .get(index as uint)
+                .map(|elem| elem.clone()),
+            Live(ref root, ref filter) => root.traverse_preorder()
+                .filter_map(|node| ElementCast::to(&node))
+                .filter(|elem| filter.filter(elem, root))
+                .nth(index as uint).clone()
         }
     }
 
@@ -86,9 +113,20 @@ impl HTMLCollection {
         }
 
         // Step 2.
-        self.elements.iter().find(|elem| {
-            elem.get_string_attribute("name") == key || elem.get_string_attribute("id") == key
-        }).map(|maybe_elem| maybe_elem.clone())
+        match self.collection {
+            Static(ref elems) => elems.iter()
+                .find(|elem| {
+                    elem.get_string_attribute("name") == key ||
+                    elem.get_string_attribute("id") == key })
+                .map(|maybe_elem| maybe_elem.clone()),
+            Live(ref root, ref filter) => root.traverse_preorder()
+                .filter_map(|node| ElementCast::to(&node))
+                .filter(|elem| filter.filter(elem, root))
+                .find(|elem| {
+                    elem.get_string_attribute("name") == key ||
+                    elem.get_string_attribute("id") == key })
+                .map(|maybe_elem| maybe_elem.clone())
+        }
     }
 }
 
diff --git a/src/components/script/dom/htmlformelement.rs b/src/components/script/dom/htmlformelement.rs
index a1ca7d5dc2f..cabb71cc1ea 100644
--- a/src/components/script/dom/htmlformelement.rs
+++ b/src/components/script/dom/htmlformelement.rs
@@ -9,7 +9,7 @@ use dom::bindings::error::ErrorResult;
 use dom::document::Document;
 use dom::element::{Element, HTMLFormElementTypeId};
 use dom::eventtarget::{EventTarget, NodeTargetTypeId};
-use dom::htmlcollection::HTMLCollection;
+use dom::htmlcollection::{HTMLCollection, Static};
 use dom::htmlelement::HTMLElement;
 use dom::node::{Node, ElementNodeTypeId};
 use servo_util::str::DOMString;
@@ -118,7 +118,7 @@ impl HTMLFormElement {
         // FIXME: https://github.com/mozilla/servo/issues/1844
         let doc = self.htmlelement.element.node.owner_doc();
         let doc = doc.get();
-        HTMLCollection::new(&doc.window, ~[])
+        HTMLCollection::new(&doc.window, Static(~[]))
     }
 
     pub fn Length(&self) -> i32 {
diff --git a/src/components/script/dom/htmlmapelement.rs b/src/components/script/dom/htmlmapelement.rs
index d343073a278..c439225362c 100644
--- a/src/components/script/dom/htmlmapelement.rs
+++ b/src/components/script/dom/htmlmapelement.rs
@@ -9,7 +9,7 @@ use dom::bindings::error::ErrorResult;
 use dom::document::Document;
 use dom::element::HTMLMapElementTypeId;
 use dom::eventtarget::{EventTarget, NodeTargetTypeId};
-use dom::htmlcollection::HTMLCollection;
+use dom::htmlcollection::{HTMLCollection, Static};
 use dom::htmlelement::HTMLElement;
 use dom::node::{Node, ElementNodeTypeId};
 use servo_util::str::DOMString;
@@ -54,6 +54,6 @@ impl HTMLMapElement {
         // FIXME: https://github.com/mozilla/servo/issues/1845
         let doc = self.htmlelement.element.node.owner_doc();
         let doc = doc.get();
-        HTMLCollection::new(&doc.window, ~[])
+        HTMLCollection::new(&doc.window, Static(~[]))
     }
 }