diff --git a/Cargo.lock b/Cargo.lock
index af19dedf..c221679d 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -527,6 +527,7 @@ dependencies = [
  "html5ever",
  "indexmap",
  "matches",
+ "once_cell",
  "selectors",
  "smallvec",
  "tendril",
diff --git a/Cargo.toml b/Cargo.toml
index 57c822be..aedc844a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -24,6 +24,7 @@ smallvec = "1.10.0"
 tendril = "0.4.3"
 ahash = "0.8"
 indexmap = { version = "1.9.2", optional = true }
+once_cell = "1.0"
 
 [dependencies.getopts]
 version = "0.2.21"
diff --git a/src/element_ref/element.rs b/src/element_ref/element.rs
index 6998dc1f..07a76c79 100644
--- a/src/element_ref/element.rs
+++ b/src/element_ref/element.rs
@@ -106,8 +106,8 @@ impl<'a> Element for ElementRef<'a> {
     }
 
     fn has_id(&self, id: &CssLocalName, case_sensitivity: CaseSensitivity) -> bool {
-        match self.value().id {
-            Some(ref val) => case_sensitivity.eq(id.0.as_bytes(), val.as_bytes()),
+        match self.value().id() {
+            Some(val) => case_sensitivity.eq(id.0.as_bytes(), val.as_bytes()),
             None => false,
         }
     }
diff --git a/src/node.rs b/src/node.rs
index 428a0553..f5273262 100644
--- a/src/node.rs
+++ b/src/node.rs
@@ -2,16 +2,15 @@
 
 #[cfg(not(feature = "deterministic"))]
 use ahash::AHashMap as HashMap;
-use ahash::AHashSet as HashSet;
 #[cfg(not(feature = "deterministic"))]
 use std::collections::hash_map;
-use std::collections::hash_set;
 use std::fmt;
 use std::ops::Deref;
+use std::slice::Iter as SliceIter;
 
-use crate::StrTendril;
+use crate::{CaseSensitivity, StrTendril};
 use html5ever::{Attribute, LocalName, QualName};
-use selectors::attr::CaseSensitivity;
+use once_cell::unsync::OnceCell;
 
 /// An HTML node.
 // `Element` is usally the most common variant and hence boxing it
@@ -228,41 +227,27 @@ pub struct Element {
     /// The element name.
     pub name: QualName,
 
-    /// The element ID.
-    pub id: Option<LocalName>,
-
-    /// The element classes.
-    pub classes: HashSet<LocalName>,
-
     /// The element attributes.
     pub attrs: Attributes,
+
+    id: OnceCell<Option<StrTendril>>,
+
+    classes: OnceCell<Vec<LocalName>>,
 }
 
 impl Element {
     #[doc(hidden)]
     pub fn new(name: QualName, attributes: Vec<Attribute>) -> Self {
-        let mut classes: HashSet<LocalName> = HashSet::new();
-        let mut attrs = Attributes::with_capacity(attributes.len());
-        let mut id: Option<LocalName> = None;
-
-        for a in attributes {
-            match a.name.local.deref() {
-                "id" => {
-                    id = Some(LocalName::from(a.value.deref()));
-                }
-                "class" => {
-                    classes.extend(a.value.deref().split_whitespace().map(LocalName::from));
-                }
-                _ => (),
-            };
-            attrs.insert(a.name, crate::tendril_util::make(a.value));
-        }
+        let attrs = attributes
+            .into_iter()
+            .map(|a| (a.name, crate::tendril_util::make(a.value)))
+            .collect();
 
         Element {
             attrs,
             name,
-            id,
-            classes,
+            id: OnceCell::new(),
+            classes: OnceCell::new(),
         }
     }
 
@@ -273,7 +258,14 @@ impl Element {
 
     /// Returns the element ID.
     pub fn id(&self) -> Option<&str> {
-        self.id.as_deref()
+        self.id
+            .get_or_init(|| {
+                self.attrs
+                    .iter()
+                    .find(|(name, _)| name.local.as_ref() == "id")
+                    .map(|(_, value)| value.clone())
+            })
+            .as_deref()
     }
 
     /// Returns true if element has the class.
@@ -284,8 +276,22 @@ impl Element {
 
     /// Returns an iterator over the element's classes.
     pub fn classes(&self) -> Classes {
+        let classes = self.classes.get_or_init(|| {
+            let mut classes: Vec<LocalName> = self
+                .attrs
+                .iter()
+                .filter(|(name, _)| name.local.as_ref() == "class")
+                .flat_map(|(_, value)| value.split_whitespace().map(LocalName::from))
+                .collect();
+
+            classes.sort_unstable();
+            classes.dedup();
+
+            classes
+        });
+
         Classes {
-            inner: self.classes.iter(),
+            inner: classes.iter(),
         }
     }
 
@@ -307,7 +313,7 @@ impl Element {
 #[allow(missing_debug_implementations)]
 #[derive(Clone)]
 pub struct Classes<'a> {
-    inner: hash_set::Iter<'a, LocalName>,
+    inner: SliceIter<'a, LocalName>,
 }
 
 impl<'a> Iterator for Classes<'a> {