diff --git a/components/layout/flow.rs b/components/layout/flow.rs
index f4c545ec730..a2a05761438 100644
--- a/components/layout/flow.rs
+++ b/components/layout/flow.rs
@@ -765,6 +765,10 @@ impl<'a> Iterator for AbsoluteDescendantIter<'a> {
     fn next(&mut self) -> Option<&'a mut Flow> {
         self.iter.next().map(|info| FlowRef::deref_mut(&mut info.flow))
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
 }
 
 pub type AbsoluteDescendantOffsetIter<'a> = Zip<AbsoluteDescendantIter<'a>, IterMut<'a, Au>>;
diff --git a/components/layout/inline.rs b/components/layout/inline.rs
index f07943fd321..c38693c3340 100644
--- a/components/layout/inline.rs
+++ b/components/layout/inline.rs
@@ -979,6 +979,10 @@ impl InlineFlow {
                         self.iter.next()
                     }
                 }
+
+                fn size_hint(&self) -> (usize, Option<usize>) {
+                    self.iter.size_hint()
+                }
             }
 
             // If the bidi embedding direction is opposite the layout direction, lay out this
diff --git a/components/script/dom/htmlselectelement.rs b/components/script/dom/htmlselectelement.rs
index fd5a864862a..9db95316297 100755
--- a/components/script/dom/htmlselectelement.rs
+++ b/components/script/dom/htmlselectelement.rs
@@ -440,4 +440,12 @@ impl<I, J, K, T> Iterator for Choice3<I, J, K>
             Choice3::Third(ref mut k) => k.next(),
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match *self {
+            Choice3::First(ref i) => i.size_hint(),
+            Choice3::Second(ref j) => j.size_hint(),
+            Choice3::Third(ref k) => k.size_hint(),
+        }
+    }
 }
diff --git a/components/script/dom/servoparser/mod.rs b/components/script/dom/servoparser/mod.rs
index 241179ed624..d0e08664e73 100644
--- a/components/script/dom/servoparser/mod.rs
+++ b/components/script/dom/servoparser/mod.rs
@@ -487,6 +487,10 @@ impl<I> Iterator for FragmentParsingResult<I>
         next.remove_self();
         Some(next)
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
 }
 
 #[derive(HeapSizeOf, JSTraceable, PartialEq)]
diff --git a/components/style/font_face.rs b/components/style/font_face.rs
index c12934a3b06..30eb242a6d4 100644
--- a/components/style/font_face.rs
+++ b/components/style/font_face.rs
@@ -170,6 +170,10 @@ impl Iterator for EffectiveSources {
     fn next(&mut self) -> Option<Source> {
         self.0.pop()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.0.len(), Some(self.0.len()))
+    }
 }
 
 struct FontFaceRuleParser<'a, 'b: 'a> {
diff --git a/components/style/properties/declaration_block.rs b/components/style/properties/declaration_block.rs
index a87b87fe9db..15573cc6aef 100644
--- a/components/style/properties/declaration_block.rs
+++ b/components/style/properties/declaration_block.rs
@@ -99,6 +99,10 @@ impl<'a> Iterator for DeclarationImportanceIterator<'a> {
         self.iter.next().map(|(decl, important)|
             (decl, if important { Importance::Important } else { Importance::Normal }))
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
 }
 
 impl<'a> DoubleEndedIterator for DeclarationImportanceIterator<'a> {
diff --git a/components/style/stylesheet_set.rs b/components/style/stylesheet_set.rs
index 328237ccbae..4b8b177b554 100644
--- a/components/style/stylesheet_set.rs
+++ b/components/style/stylesheet_set.rs
@@ -45,6 +45,10 @@ where
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next().map(|entry| &entry.sheet)
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 /// An iterator over the flattened view of the stylesheet collections.
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index cad3db03aa7..a6c828b14b8 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -198,6 +198,10 @@ impl<'a, 'cx, 'cx_a: 'cx, S: ToComputedValue + 'a> Iterator for ComputedVecIter<
             None
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.values.len(), Some(self.values.len()))
+    }
 }
 
 /// A trait to represent the conversion between computed and specified values.