diff --git a/components/selectors/visitor.rs b/components/selectors/visitor.rs
index 07f121fe82f..51d335c6a74 100644
--- a/components/selectors/visitor.rs
+++ b/components/selectors/visitor.rs
@@ -37,7 +37,7 @@ pub trait SelectorVisitor {
     /// Visits a complex selector.
     ///
     /// Gets the combinator to the right of the selector, or `None` if the
-    /// selector is the leftmost one.
+    /// selector is the rightmost one.
     fn visit_complex_selector(&mut self,
                               _: SelectorIter<Self::Impl>,
                               _combinator_to_right: Option<Combinator>)
diff --git a/components/style/selector_map.rs b/components/style/selector_map.rs
index 497baf2f914..846d7972811 100644
--- a/components/style/selector_map.rs
+++ b/components/style/selector_map.rs
@@ -159,6 +159,12 @@ impl SelectorMap<Rule> {
                     |block| (block.specificity, block.source_order));
     }
 
+    /// Check whether we have rules for the given id
+    #[inline]
+    pub fn has_rules_for_id(&self, id: &Atom) -> bool {
+        self.id_hash.get(id).is_some()
+    }
+
     /// Append to `rule_list` all universal Rules (rules with selector `*|*`) in
     /// `self` sorted by specificity and source order.
     pub fn get_universal_rules(&self,
diff --git a/components/style/sharing/checks.rs b/components/style/sharing/checks.rs
index d8ce6565e62..581022392d6 100644
--- a/components/style/sharing/checks.rs
+++ b/components/style/sharing/checks.rs
@@ -6,6 +6,7 @@
 //! quickly whether it's worth to share style, and whether two different
 //! elements can indeed share the same style.
 
+use Atom;
 use bloom::StyleBloom;
 use context::{SelectorFlagsMap, SharedStyleContext};
 use dom::TElement;
@@ -123,3 +124,28 @@ pub fn revalidate<E>(target: &mut StyleSharingTarget<E>,
 
     for_element == for_candidate
 }
+
+/// Checks whether we might have rules for either of the two ids.
+#[inline]
+pub fn may_have_rules_for_ids(shared_context: &SharedStyleContext,
+                              element_id: Option<&Atom>,
+                              candidate_id: Option<&Atom>) -> bool
+{
+    // We shouldn't be called unless the ids are different.
+    debug_assert!(element_id.is_some() || candidate_id.is_some());
+    let stylist = &shared_context.stylist;
+
+    let may_have_rules_for_element = match element_id {
+        Some(id) => stylist.may_have_rules_for_id(id),
+        None => false
+    };
+
+    if may_have_rules_for_element {
+        return true;
+    }
+
+    match candidate_id {
+        Some(id) => stylist.may_have_rules_for_id(id),
+        None => false
+    }
+}
diff --git a/components/style/sharing/mod.rs b/components/style/sharing/mod.rs
index 5a884bef138..a6598ec6769 100644
--- a/components/style/sharing/mod.rs
+++ b/components/style/sharing/mod.rs
@@ -509,11 +509,6 @@ impl<E: TElement> StyleSharingCandidateCache<E> {
             return StyleSharingResult::CannotShare;
         }
 
-        if target.get_id().is_some() {
-            debug!("{:?} Cannot share style: element has id", target.element);
-            return StyleSharingResult::CannotShare
-        }
-
         let mut should_clear_cache = false;
         for (i, candidate) in self.iter_mut().enumerate() {
             let sharing_result =
@@ -632,8 +627,14 @@ impl<E: TElement> StyleSharingCandidateCache<E> {
             miss!(State)
         }
 
-        if target.get_id() != candidate.element.get_id() {
-            miss!(IdAttr)
+        let element_id = target.element.get_id();
+        let candidate_id = candidate.element.get_id();
+        if element_id != candidate_id {
+            // It's possible that there are no styles for either id.
+            if checks::may_have_rules_for_ids(shared, element_id.as_ref(),
+                                              candidate_id.as_ref()) {
+                miss!(IdAttr)
+            }
         }
 
         if !checks::have_same_style_attribute(target, candidate) {
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 5e8948c23fc..5df0bf2717b 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -146,6 +146,15 @@ pub struct Stylist {
     /// when an irrelevant element state bit changes.
     state_dependencies: ElementState,
 
+    /// The ids that appear in the rightmost complex selector of selectors (and
+    /// hence in our selector maps).  Used to determine when sharing styles is
+    /// safe: we disallow style sharing for elements whose id matches this
+    /// filter, and hence might be in one of our selector maps.
+    ///
+    /// FIXME(bz): This doesn't really need to be a counting Blooom filter.
+    #[cfg_attr(feature = "servo", ignore_heap_size_of = "just an array")]
+    mapped_ids: BloomFilter,
+
     /// Selectors that require explicit cache revalidation (i.e. which depend
     /// on state that is not otherwise visible to the cache, like attributes or
     /// tree-structural state like child index and pseudos).
@@ -249,6 +258,7 @@ impl Stylist {
             attribute_dependencies: BloomFilter::new(),
             style_attribute_dependency: false,
             state_dependencies: ElementState::empty(),
+            mapped_ids: BloomFilter::new(),
             selectors_for_cache_revalidation: SelectorMap::new(),
             num_selectors: 0,
             num_declarations: 0,
@@ -323,6 +333,7 @@ impl Stylist {
         self.attribute_dependencies.clear();
         self.style_attribute_dependency = false;
         self.state_dependencies = ElementState::empty();
+        self.mapped_ids.clear();
         self.selectors_for_cache_revalidation = SelectorMap::new();
         self.num_selectors = 0;
         self.num_declarations = 0;
@@ -468,6 +479,9 @@ impl Stylist {
                             style_attribute_dependency: &mut self.style_attribute_dependency,
                             state_dependencies: &mut self.state_dependencies,
                         });
+                        selector.visit(&mut MappedIdVisitor {
+                            mapped_ids: &mut self.mapped_ids,
+                        });
                     }
                     self.rules_source_order += 1;
                 }
@@ -1069,6 +1083,13 @@ impl Stylist {
         debug!("push_applicable_declarations: shareable: {:?}", context.relations);
     }
 
+    /// Given an id, returns whether there might be any rules for that id in any
+    /// of our rule maps.
+    #[inline]
+    pub fn may_have_rules_for_id(&self, id: &Atom) -> bool {
+        self.mapped_ids.might_contain(id)
+    }
+
     /// Return whether the device is dirty, that is, whether the screen size or
     /// media type have changed (for now).
     #[inline]
@@ -1232,6 +1253,37 @@ impl<'a> SelectorVisitor for AttributeAndStateDependencyVisitor<'a> {
     }
 }
 
+/// Visitor to collect ids that appear in the rightmost portion of selectors.
+struct MappedIdVisitor<'a> {
+    mapped_ids: &'a mut BloomFilter,
+}
+
+impl<'a> SelectorVisitor for MappedIdVisitor<'a> {
+    type Impl = SelectorImpl;
+
+    /// We just want to insert all the ids we find into mapped_ids.
+    fn visit_simple_selector(&mut self, s: &Component<SelectorImpl>) -> bool {
+        if let Component::ID(ref id) = *s {
+            self.mapped_ids.insert(id);
+        }
+        true
+    }
+
+    /// We want to stop as soon as we've moved off the rightmost ComplexSelector
+    /// that is not a psedo-element.  That can be detected by a
+    /// visit_complex_selector call with a combinator other than None and
+    /// PseudoElement.  Importantly, this call happens before we visit any of
+    /// the simple selectors in that ComplexSelector.
+    fn visit_complex_selector(&mut self,
+                              _: SelectorIter<SelectorImpl>,
+                              combinator: Option<Combinator>) -> bool {
+        match combinator {
+            None | Some(Combinator::PseudoElement) => true,
+            _ => false,
+        }
+    }
+}
+
 /// Visitor determine whether a selector requires cache revalidation.
 ///
 /// Note that we just check simple selectors and eagerly return when the first