diff --git a/components/style/invalidation/element/invalidation_map.rs b/components/style/invalidation/element/invalidation_map.rs
index 153c13bc149..0bba423a17f 100644
--- a/components/style/invalidation/element/invalidation_map.rs
+++ b/components/style/invalidation/element/invalidation_map.rs
@@ -11,7 +11,7 @@ use crate::selector_map::{
 };
 use crate::selector_parser::SelectorImpl;
 use crate::AllocErr;
-use crate::{Atom, LocalName, Namespace};
+use crate::{Atom, LocalName, Namespace, ShrinkIfNeeded};
 use selectors::attr::NamespaceConstraint;
 use selectors::parser::{Combinator, Component};
 use selectors::parser::{Selector, SelectorIter};
@@ -237,6 +237,14 @@ impl InvalidationMap {
         self.other_attribute_affecting_selectors.clear();
     }
 
+    /// Shrink the capacity of hash maps if needed.
+    pub fn shrink_if_needed(&mut self) {
+        self.class_to_selector.shrink_if_needed();
+        self.id_to_selector.shrink_if_needed();
+        self.state_affecting_selectors.shrink_if_needed();
+        self.other_attribute_affecting_selectors.shrink_if_needed();
+    }
+
     /// Adds a selector to this `InvalidationMap`.  Returns Err(..) to
     /// signify OOM.
     pub fn note_selector(
diff --git a/components/style/invalidation/stylesheets.rs b/components/style/invalidation/stylesheets.rs
index c18cc3cd242..53130de7a2e 100644
--- a/components/style/invalidation/stylesheets.rs
+++ b/components/style/invalidation/stylesheets.rs
@@ -18,7 +18,7 @@ use crate::shared_lock::SharedRwLockReadGuard;
 use crate::stylesheets::{CssRule, StylesheetInDocument};
 use crate::stylesheets::{EffectiveRules, EffectiveRulesIterator};
 use crate::values::AtomIdent;
-use crate::Atom;
+use crate::{Atom, ShrinkIfNeeded};
 use crate::LocalName as SelectorLocalName;
 use selectors::parser::{Component, LocalName, Selector};
 
@@ -119,6 +119,15 @@ impl StylesheetInvalidationSet {
         self.fully_invalid = true;
     }
 
+    fn shrink_if_needed(&mut self) {
+        if self.fully_invalid {
+            return;
+        }
+        self.classes.shrink_if_needed();
+        self.ids.shrink_if_needed();
+        self.local_names.shrink_if_needed();
+    }
+
     /// Analyze the given stylesheet, and collect invalidations from their
     /// rules, in order to avoid doing a full restyle when we style the document
     /// next time.
@@ -149,6 +158,8 @@ impl StylesheetInvalidationSet {
             }
         }
 
+        self.shrink_if_needed();
+
         debug!(" > resulting class invalidations: {:?}", self.classes);
         debug!(" > resulting id invalidations: {:?}", self.ids);
         debug!(
diff --git a/components/style/lib.rs b/components/style/lib.rs
index 00c8359b5f4..2967fbcf7ce 100644
--- a/components/style/lib.rs
+++ b/components/style/lib.rs
@@ -157,6 +157,8 @@ pub use style_traits::arc_slice::ArcSlice;
 pub use style_traits::owned_slice::OwnedSlice;
 pub use style_traits::owned_str::OwnedStr;
 
+use std::hash::{Hash, BuildHasher};
+
 /// The CSS properties supported by the style system.
 /// Generated from the properties.mako.rs template by build.rs
 #[macro_use]
@@ -286,3 +288,44 @@ impl From<std::collections::TryReserveError> for AllocErr {
         Self
     }
 }
+
+/// Shrink the capacity of the collection if needed.
+pub (crate) trait ShrinkIfNeeded {
+    fn shrink_if_needed(&mut self);
+}
+
+/// We shrink the capacity of a collection if we're wasting more than a 25% of
+/// its capacity, and if the collection is arbitrarily big enough
+/// (>= CAPACITY_THRESHOLD entries).
+#[inline]
+fn should_shrink(len: usize, capacity: usize) -> bool {
+    const CAPACITY_THRESHOLD: usize = 64;
+    capacity >= CAPACITY_THRESHOLD && len + capacity / 4 < capacity
+}
+
+impl<K, V, H> ShrinkIfNeeded for std::collections::HashMap<K, V, H>
+where
+    K: Eq + Hash,
+    H: BuildHasher,
+{
+    fn shrink_if_needed(&mut self) {
+        if should_shrink(self.len(), self.capacity()) {
+            self.shrink_to_fit();
+        }
+    }
+}
+
+impl<T, H> ShrinkIfNeeded for std::collections::HashSet<T, H>
+where
+    T: Eq + Hash,
+    H: BuildHasher,
+{
+    fn shrink_if_needed(&mut self) {
+        if should_shrink(self.len(), self.capacity()) {
+            self.shrink_to_fit();
+        }
+    }
+}
+
+// TODO(emilio): Measure and see if we're wasting a lot of memory on Vec /
+// SmallVec, and if so consider shrinking those as well.
diff --git a/components/style/selector_map.rs b/components/style/selector_map.rs
index 0aa8cefc406..8a575ca8386 100644
--- a/components/style/selector_map.rs
+++ b/components/style/selector_map.rs
@@ -12,7 +12,7 @@ use crate::rule_tree::CascadeLevel;
 use crate::selector_parser::SelectorImpl;
 use crate::stylist::{CascadeData, Rule};
 use crate::AllocErr;
-use crate::{Atom, LocalName, Namespace, WeakAtom};
+use crate::{Atom, LocalName, Namespace, ShrinkIfNeeded, WeakAtom};
 use precomputed_hash::PrecomputedHash;
 use selectors::matching::{matches_selector, ElementSelectorFlags, MatchingContext};
 use selectors::parser::{Combinator, Component, SelectorIter};
@@ -122,10 +122,7 @@ impl<T: 'static> Default for SelectorMap<T> {
     }
 }
 
-// FIXME(Manishearth) the 'static bound can be removed when
-// our HashMap fork (hashglobe) is able to use NonZero,
-// or when stdlib gets fallible collections
-impl<T: 'static> SelectorMap<T> {
+impl<T> SelectorMap<T> {
     /// Trivially constructs an empty `SelectorMap`.
     pub fn new() -> Self {
         SelectorMap {
@@ -152,6 +149,15 @@ impl<T: 'static> SelectorMap<T> {
         ret
     }
 
+    /// Shrink the capacity of the map if needed.
+    pub fn shrink_if_needed(&mut self) {
+        self.id_hash.shrink_if_needed();
+        self.class_hash.shrink_if_needed();
+        self.attribute_hash.shrink_if_needed();
+        self.local_name_hash.shrink_if_needed();
+        self.namespace_hash.shrink_if_needed();
+    }
+
     /// Clears the hashmap retaining storage.
     pub fn clear(&mut self) {
         self.root.clear();
@@ -715,26 +721,28 @@ fn find_bucket<'a>(
 
 /// Wrapper for PrecomputedHashMap that does ASCII-case-insensitive lookup in quirks mode.
 #[derive(Clone, Debug, MallocSizeOf)]
-pub struct MaybeCaseInsensitiveHashMap<K: PrecomputedHash + Hash + Eq, V: 'static>(
+pub struct MaybeCaseInsensitiveHashMap<K: PrecomputedHash + Hash + Eq, V>(
     PrecomputedHashMap<K, V>,
 );
 
-impl<V: 'static> Default for MaybeCaseInsensitiveHashMap<Atom, V> {
+impl<V> Default for MaybeCaseInsensitiveHashMap<Atom, V> {
     #[inline]
     fn default() -> Self {
         MaybeCaseInsensitiveHashMap(PrecomputedHashMap::default())
     }
 }
 
-// FIXME(Manishearth) the 'static bound can be removed when
-// our HashMap fork (hashglobe) is able to use NonZero,
-// or when stdlib gets fallible collections
-impl<V: 'static> MaybeCaseInsensitiveHashMap<Atom, V> {
+impl<V> MaybeCaseInsensitiveHashMap<Atom, V> {
     /// Empty map
     pub fn new() -> Self {
         Self::default()
     }
 
+    /// Shrink the capacity of the map if needed.
+    pub fn shrink_if_needed(&mut self) {
+        self.0.shrink_if_needed()
+    }
+
     /// HashMap::try_entry
     pub fn try_entry(
         &mut self,
diff --git a/components/style/selector_parser.rs b/components/style/selector_parser.rs
index d28e333da04..f29cab9735d 100644
--- a/components/style/selector_parser.rs
+++ b/components/style/selector_parser.rs
@@ -170,9 +170,14 @@ impl<T> PerPseudoElementMap<T> {
     }
 
     /// Get an iterator for the entries.
-    pub fn iter(&self) -> ::std::slice::Iter<Option<T>> {
+    pub fn iter(&self) -> std::slice::Iter<Option<T>> {
         self.entries.iter()
     }
+
+    /// Get a mutable iterator for the entries.
+    pub fn iter_mut(&mut self) -> std::slice::IterMut<Option<T>> {
+        self.entries.iter_mut()
+    }
 }
 
 /// Values for the :dir() pseudo class
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 3b170e0e339..14f88c4fe22 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -41,7 +41,7 @@ use crate::stylesheets::{
 use crate::stylesheets::{StyleRule, StylesheetContents, StylesheetInDocument};
 use crate::thread_state::{self, ThreadState};
 use crate::AllocErr;
-use crate::{Atom, LocalName, Namespace, WeakAtom};
+use crate::{Atom, LocalName, Namespace, ShrinkIfNeeded, WeakAtom};
 use fxhash::FxHashMap;
 use malloc_size_of::MallocSizeOf;
 #[cfg(feature = "gecko")]
@@ -291,7 +291,7 @@ impl CascadeDataCacheEntry for UserAgentCascadeData {
             )?;
         }
 
-        new_data.cascade_data.compute_layer_order();
+        new_data.cascade_data.did_finish_rebuild();
 
         Ok(Arc::new(new_data))
     }
@@ -1978,6 +1978,15 @@ impl ElementAndPseudoRules {
         self.element_map.clear();
         self.pseudos_map.clear();
     }
+
+    fn shrink_if_needed(&mut self) {
+        self.element_map.shrink_if_needed();
+        for pseudo in self.pseudos_map.iter_mut() {
+            if let Some(ref mut pseudo) = pseudo {
+                pseudo.shrink_if_needed();
+            }
+        }
+    }
 }
 
 impl PartElementAndPseudoRules {
@@ -2164,7 +2173,7 @@ impl CascadeData {
             result.is_ok()
         });
 
-        self.compute_layer_order();
+        self.did_finish_rebuild();
 
         result
     }
@@ -2232,6 +2241,27 @@ impl CascadeData {
         self.layers[id.0 as usize].order
     }
 
+    fn did_finish_rebuild(&mut self) {
+        self.shrink_maps_if_needed();
+        self.compute_layer_order();
+    }
+
+    fn shrink_maps_if_needed(&mut self) {
+        self.normal_rules.shrink_if_needed();
+        if let Some(ref mut host_rules) = self.host_rules {
+            host_rules.shrink_if_needed();
+        }
+        if let Some(ref mut slotted_rules) = self.slotted_rules {
+            slotted_rules.shrink_if_needed();
+        }
+        self.invalidation_map.shrink_if_needed();
+        self.attribute_dependencies.shrink_if_needed();
+        self.mapped_ids.shrink_if_needed();
+        self.layer_id.shrink_if_needed();
+        self.selectors_for_cache_revalidation.shrink_if_needed();
+
+    }
+
     fn compute_layer_order(&mut self) {
         debug_assert_ne!(
             self.layers.len(),