diff --git a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
index ef7e501214..ac5ca7d1c3 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
+++ b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
@@ -439,7 +439,7 @@ static void set_property_expanding_shorthands(StyleProperties& style, CSS::Prope
     style.set_property(property_id, value);
 }
 
-bool StyleComputer::expand_unresolved_values(DOM::Element& element, StringView property_name, HashMap<String, NonnullRefPtr<PropertyDependencyNode>>& dependencies, Vector<StyleComponentValueRule> const& source, Vector<StyleComponentValueRule>& dest, size_t source_start_index, HashMap<String, StyleProperty const*> const& custom_properties) const
+bool StyleComputer::expand_unresolved_values(DOM::Element& element, StringView property_name, HashMap<FlyString, NonnullRefPtr<PropertyDependencyNode>>& dependencies, Vector<StyleComponentValueRule> const& source, Vector<StyleComponentValueRule>& dest, size_t source_start_index, HashMap<FlyString, StyleProperty const*> const& custom_properties) const
 {
     // FIXME: Do this better!
     // We build a copy of the tree of StyleComponentValueRules, with all var()s replaced with their contents.
@@ -532,14 +532,14 @@ bool StyleComputer::expand_unresolved_values(DOM::Element& element, StringView p
     return true;
 }
 
-RefPtr<StyleValue> StyleComputer::resolve_unresolved_style_value(DOM::Element& element, PropertyID property_id, UnresolvedStyleValue const& unresolved, HashMap<String, StyleProperty const*> const& custom_properties) const
+RefPtr<StyleValue> StyleComputer::resolve_unresolved_style_value(DOM::Element& element, PropertyID property_id, UnresolvedStyleValue const& unresolved, HashMap<FlyString, StyleProperty const*> const& custom_properties) const
 {
     // Unresolved always contains a var(), unless it is a custom property's value, in which case we shouldn't be trying
     // to produce a different StyleValue from it.
     VERIFY(unresolved.contains_var());
 
     Vector<StyleComponentValueRule> expanded_values;
-    HashMap<String, NonnullRefPtr<PropertyDependencyNode>> dependencies;
+    HashMap<FlyString, NonnullRefPtr<PropertyDependencyNode>> dependencies;
     if (!expand_unresolved_values(element, string_from_property_id(property_id), dependencies, unresolved.values(), expanded_values, 0, custom_properties))
         return {};
 
@@ -549,7 +549,7 @@ RefPtr<StyleValue> StyleComputer::resolve_unresolved_style_value(DOM::Element& e
     return {};
 }
 
-void StyleComputer::cascade_declarations(StyleProperties& style, DOM::Element& element, Vector<MatchingRule> const& matching_rules, CascadeOrigin cascade_origin, Important important, HashMap<String, StyleProperty const*> const& custom_properties) const
+void StyleComputer::cascade_declarations(StyleProperties& style, DOM::Element& element, Vector<MatchingRule> const& matching_rules, CascadeOrigin cascade_origin, Important important, HashMap<FlyString, StyleProperty const*> const& custom_properties) const
 {
     for (auto const& match : matching_rules) {
         for (auto const& property : verify_cast<PropertyOwningCSSStyleDeclaration>(match.rule->declaration()).properties()) {
@@ -575,9 +575,9 @@ void StyleComputer::cascade_declarations(StyleProperties& style, DOM::Element& e
     }
 }
 
-static HashMap<String, StyleProperty const*> cascade_custom_properties(DOM::Element& element, Vector<MatchingRule> const& matching_rules)
+static HashMap<FlyString, StyleProperty const*> cascade_custom_properties(DOM::Element& element, Vector<MatchingRule> const& matching_rules)
 {
-    HashMap<String, StyleProperty const*> custom_properties;
+    HashMap<FlyString, StyleProperty const*> custom_properties;
 
     if (auto* parent_element = element.parent_element()) {
         for (auto const& it : parent_element->custom_properties())
diff --git a/Userland/Libraries/LibWeb/CSS/StyleComputer.h b/Userland/Libraries/LibWeb/CSS/StyleComputer.h
index dd05dc7582..11714844db 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleComputer.h
+++ b/Userland/Libraries/LibWeb/CSS/StyleComputer.h
@@ -78,8 +78,8 @@ private:
 
     void compute_defaulted_property_value(StyleProperties&, DOM::Element const*, CSS::PropertyID, Optional<CSS::Selector::PseudoElement>) const;
 
-    RefPtr<StyleValue> resolve_unresolved_style_value(DOM::Element&, PropertyID, UnresolvedStyleValue const&, HashMap<String, StyleProperty const*> const&) const;
-    bool expand_unresolved_values(DOM::Element&, StringView property_name, HashMap<String, NonnullRefPtr<PropertyDependencyNode>>& dependencies, Vector<StyleComponentValueRule> const& source, Vector<StyleComponentValueRule>& dest, size_t source_start_index, HashMap<String, StyleProperty const*> const& custom_properties) const;
+    RefPtr<StyleValue> resolve_unresolved_style_value(DOM::Element&, PropertyID, UnresolvedStyleValue const&, HashMap<FlyString, StyleProperty const*> const&) const;
+    bool expand_unresolved_values(DOM::Element&, StringView property_name, HashMap<FlyString, NonnullRefPtr<PropertyDependencyNode>>& dependencies, Vector<StyleComponentValueRule> const& source, Vector<StyleComponentValueRule>& dest, size_t source_start_index, HashMap<FlyString, StyleProperty const*> const& custom_properties) const;
 
     template<typename Callback>
     void for_each_stylesheet(CascadeOrigin, Callback) const;
@@ -89,7 +89,7 @@ private:
         Vector<MatchingRule> author_rules;
     };
 
-    void cascade_declarations(StyleProperties&, DOM::Element&, Vector<MatchingRule> const&, CascadeOrigin, Important important, HashMap<String, StyleProperty const*> const&) const;
+    void cascade_declarations(StyleProperties&, DOM::Element&, Vector<MatchingRule> const&, CascadeOrigin, Important important, HashMap<FlyString, StyleProperty const*> const&) const;
 
     void build_rule_cache();
     void build_rule_cache_if_needed() const;
diff --git a/Userland/Libraries/LibWeb/DOM/Element.h b/Userland/Libraries/LibWeb/DOM/Element.h
index 78d4bd7ac9..e1bdb47ab6 100644
--- a/Userland/Libraries/LibWeb/DOM/Element.h
+++ b/Userland/Libraries/LibWeb/DOM/Element.h
@@ -118,8 +118,8 @@ public:
         m_custom_properties.set(move(custom_property_name), move(style_property));
     }
 
-    HashMap<String, CSS::StyleProperty> const& custom_properties() const { return m_custom_properties; }
-    HashMap<String, CSS::StyleProperty>& custom_properties() { return m_custom_properties; }
+    HashMap<FlyString, CSS::StyleProperty> const& custom_properties() const { return m_custom_properties; }
+    HashMap<FlyString, CSS::StyleProperty>& custom_properties() { return m_custom_properties; }
 
     void queue_an_element_task(HTML::Task::Source, Function<void()>);
 
@@ -149,7 +149,7 @@ private:
     RefPtr<CSS::CSSStyleDeclaration> m_inline_style;
 
     RefPtr<CSS::StyleProperties> m_specified_css_values;
-    HashMap<String, CSS::StyleProperty> m_custom_properties;
+    HashMap<FlyString, CSS::StyleProperty> m_custom_properties;
 
     RefPtr<DOMTokenList> m_class_list;
     Vector<FlyString> m_classes;