diff --git a/Tests/LibWeb/Text/expected/all-window-properties.txt b/Tests/LibWeb/Text/expected/all-window-properties.txt
index b7e6bc14f84..f7fc93d2f55 100644
--- a/Tests/LibWeb/Text/expected/all-window-properties.txt
+++ b/Tests/LibWeb/Text/expected/all-window-properties.txt
@@ -42,6 +42,7 @@ CSSLayerBlockRule
 CSSLayerStatementRule
 CSSMediaRule
 CSSNamespaceRule
+CSSNestedDeclarations
 CSSRule
 CSSRuleList
 CSSStyleDeclaration
diff --git a/Userland/Libraries/LibWeb/CMakeLists.txt b/Userland/Libraries/LibWeb/CMakeLists.txt
index 53a4d33b258..2fb44f7ad3e 100644
--- a/Userland/Libraries/LibWeb/CMakeLists.txt
+++ b/Userland/Libraries/LibWeb/CMakeLists.txt
@@ -47,6 +47,7 @@ set(SOURCES
     CSS/CSSLayerBlockRule.cpp
     CSS/CSSLayerStatementRule.cpp
     CSS/CSSMediaRule.cpp
+    CSS/CSSNestedDeclarations.cpp
     CSS/CSSNumericType.cpp
     CSS/CSSNamespaceRule.cpp
     CSS/CSSRule.cpp
diff --git a/Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.cpp b/Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.cpp
new file mode 100644
index 00000000000..c3f61280ed3
--- /dev/null
+++ b/Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.cpp
@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CSSNestedDeclarations.h"
+#include <LibWeb/Bindings/CSSNestedDeclarationsPrototype.h>
+#include <LibWeb/Bindings/Intrinsics.h>
+
+namespace Web::CSS {
+
+JS_DEFINE_ALLOCATOR(CSSNestedDeclarations);
+
+JS::NonnullGCPtr<CSSNestedDeclarations> CSSNestedDeclarations::create(JS::Realm& realm, PropertyOwningCSSStyleDeclaration& declaration)
+{
+    return realm.heap().allocate<CSSNestedDeclarations>(realm, realm, declaration);
+}
+
+CSSNestedDeclarations::CSSNestedDeclarations(JS::Realm& realm, PropertyOwningCSSStyleDeclaration& declaration)
+    : CSSRule(realm)
+    , m_declaration(declaration)
+{
+    m_declaration->set_parent_rule(*this);
+}
+
+void CSSNestedDeclarations::initialize(JS::Realm& realm)
+{
+    Base::initialize(realm);
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(CSSNestedDeclarations);
+}
+
+void CSSNestedDeclarations::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_declaration);
+}
+
+CSSStyleDeclaration* CSSNestedDeclarations::style()
+{
+    return m_declaration;
+}
+
+String CSSNestedDeclarations::serialized() const
+{
+    // NOTE: There's no proper spec for this yet, only this note:
+    // "The CSSNestedDeclarations rule serializes as if its declaration block had been serialized directly."
+    // - https://drafts.csswg.org/css-nesting-1/#ref-for-cssnesteddeclarations%E2%91%A1
+    // So, we'll do the simple thing and hope it's good.
+    return m_declaration->serialized();
+}
+
+}
diff --git a/Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.h b/Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.h
new file mode 100644
index 00000000000..de800fe2f0b
--- /dev/null
+++ b/Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.h
@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/CSSRule.h>
+
+namespace Web::CSS {
+
+class CSSNestedDeclarations final : public CSSRule {
+    WEB_PLATFORM_OBJECT(CSSNestedDeclarations, CSSRule);
+    JS_DECLARE_ALLOCATOR(CSSNestedDeclarations);
+
+public:
+    [[nodiscard]] static JS::NonnullGCPtr<CSSNestedDeclarations> create(JS::Realm&, PropertyOwningCSSStyleDeclaration&);
+
+    virtual ~CSSNestedDeclarations() override = default;
+
+    virtual Type type() const override { return Type::NestedDeclarations; }
+    PropertyOwningCSSStyleDeclaration const& declaration() const { return m_declaration; }
+
+    CSSStyleDeclaration* style();
+
+private:
+    CSSNestedDeclarations(JS::Realm&, PropertyOwningCSSStyleDeclaration&);
+
+    virtual void initialize(JS::Realm&) override;
+    virtual void visit_edges(Cell::Visitor&) override;
+    virtual String serialized() const override;
+
+    JS::NonnullGCPtr<PropertyOwningCSSStyleDeclaration> m_declaration;
+};
+
+template<>
+inline bool CSSRule::fast_is<CSSNestedDeclarations>() const { return type() == CSSRule::Type::NestedDeclarations; }
+
+}
diff --git a/Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.idl b/Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.idl
new file mode 100644
index 00000000000..16cdac54c85
--- /dev/null
+++ b/Userland/Libraries/LibWeb/CSS/CSSNestedDeclarations.idl
@@ -0,0 +1,9 @@
+#import <CSS/CSSRule.idl>
+#import <CSS/CSSStyleDeclaration.idl>
+
+// https://drafts.csswg.org/css-nesting-1/#cssnesteddeclarations
+[Exposed=Window]
+interface CSSNestedDeclarations : CSSRule {
+    // FIXME: Should be a CSSStyleProperties, once we have that
+    [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
+};
diff --git a/Userland/Libraries/LibWeb/CSS/CSSRule.cpp b/Userland/Libraries/LibWeb/CSS/CSSRule.cpp
index 95f98e5da87..79872b5b700 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSRule.cpp
+++ b/Userland/Libraries/LibWeb/CSS/CSSRule.cpp
@@ -53,11 +53,11 @@ FlyString const& CSSRule::parent_layer_internal_qualified_name_slow_case() const
     Vector<FlyString> layer_names;
     for (auto* rule = parent_rule(); rule; rule = rule->parent_rule()) {
         switch (rule->type()) {
-        case CSSRule::Type::Import:
+        case Type::Import:
             // TODO: Handle `layer(foo)` in import rules once we implement that.
             break;
 
-        case CSSRule::Type::LayerBlock: {
+        case Type::LayerBlock: {
             auto& layer_block = static_cast<CSSLayerBlockRule const&>(*rule);
             layer_names.append(layer_block.internal_name());
             break;
@@ -65,14 +65,15 @@ FlyString const& CSSRule::parent_layer_internal_qualified_name_slow_case() const
 
             // Ignore everything else
             // Note that LayerStatement cannot have child rules so we still ignore it here.
-        case CSSRule::Type::LayerStatement:
-        case CSSRule::Type::Style:
-        case CSSRule::Type::Media:
-        case CSSRule::Type::FontFace:
-        case CSSRule::Type::Keyframes:
-        case CSSRule::Type::Keyframe:
-        case CSSRule::Type::Namespace:
-        case CSSRule::Type::Supports:
+        case Type::LayerStatement:
+        case Type::Style:
+        case Type::Media:
+        case Type::FontFace:
+        case Type::Keyframes:
+        case Type::Keyframe:
+        case Type::Namespace:
+        case Type::Supports:
+        case Type::NestedDeclarations:
             break;
         }
     }
diff --git a/Userland/Libraries/LibWeb/CSS/CSSRule.h b/Userland/Libraries/LibWeb/CSS/CSSRule.h
index 19cb0d72899..28929d41f46 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSRule.h
+++ b/Userland/Libraries/LibWeb/CSS/CSSRule.h
@@ -34,6 +34,7 @@ public:
         // AD-HOC: These are not included in the spec, but we need them internally. So, their numbers are arbitrary.
         LayerBlock = 100,
         LayerStatement = 101,
+        NestedDeclarations = 102,
     };
 
     virtual Type type() const = 0;
diff --git a/Userland/Libraries/LibWeb/CSS/CSSRuleList.cpp b/Userland/Libraries/LibWeb/CSS/CSSRuleList.cpp
index dc967d9b12c..dc3a92d6e00 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSRuleList.cpp
+++ b/Userland/Libraries/LibWeb/CSS/CSSRuleList.cpp
@@ -144,6 +144,7 @@ void CSSRuleList::for_each_effective_rule(TraversalOrder order, Function<void(We
         case CSSRule::Type::Keyframes:
         case CSSRule::Type::LayerStatement:
         case CSSRule::Type::Namespace:
+        case CSSRule::Type::NestedDeclarations:
         case CSSRule::Type::Style:
             break;
         }
@@ -192,6 +193,7 @@ bool CSSRuleList::evaluate_media_queries(HTML::Window const& window)
         case CSSRule::Type::Keyframes:
         case CSSRule::Type::LayerStatement:
         case CSSRule::Type::Namespace:
+        case CSSRule::Type::NestedDeclarations:
         case CSSRule::Type::Style:
             break;
         }
diff --git a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
index 76ac8aa0646..85ec4990bf4 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
+++ b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
@@ -2664,6 +2664,7 @@ void StyleComputer::build_qualified_layer_names_cache()
             case CSSRule::Type::Keyframes:
             case CSSRule::Type::Keyframe:
             case CSSRule::Type::Namespace:
+            case CSSRule::Type::NestedDeclarations:
             case CSSRule::Type::Supports:
                 break;
             }
diff --git a/Userland/Libraries/LibWeb/Dump.cpp b/Userland/Libraries/LibWeb/Dump.cpp
index 68bd8f0243b..05783471271 100644
--- a/Userland/Libraries/LibWeb/Dump.cpp
+++ b/Userland/Libraries/LibWeb/Dump.cpp
@@ -14,6 +14,7 @@
 #include <LibWeb/CSS/CSSLayerBlockRule.h>
 #include <LibWeb/CSS/CSSLayerStatementRule.h>
 #include <LibWeb/CSS/CSSMediaRule.h>
+#include <LibWeb/CSS/CSSNestedDeclarations.h>
 #include <LibWeb/CSS/CSSRule.h>
 #include <LibWeb/CSS/CSSStyleRule.h>
 #include <LibWeb/CSS/CSSStyleSheet.h>
@@ -641,6 +642,9 @@ void dump_rule(StringBuilder& builder, CSS::CSSRule const& rule, int indent_leve
     case CSS::CSSRule::Type::Namespace:
         dump_namespace_rule(builder, verify_cast<CSS::CSSNamespaceRule const>(rule), indent_levels);
         break;
+    case CSS::CSSRule::Type::NestedDeclarations:
+        dump_nested_declarations(builder, verify_cast<CSS::CSSNestedDeclarations const>(rule), indent_levels);
+        break;
     case CSS::CSSRule::Type::Style:
         dump_style_rule(builder, verify_cast<CSS::CSSStyleRule const>(rule), indent_levels);
         break;
@@ -780,27 +784,32 @@ void dump_supports_rule(StringBuilder& builder, CSS::CSSSupportsRule const& supp
         dump_rule(builder, rule, indent_levels + 2);
 }
 
+void dump_declaration(StringBuilder& builder, CSS::PropertyOwningCSSStyleDeclaration const& declaration, int indent_levels)
+{
+    indent(builder, indent_levels);
+    builder.appendff("Declarations ({}):\n", declaration.length());
+    for (auto& property : declaration.properties()) {
+        indent(builder, indent_levels);
+        builder.appendff("  {}: '{}'", CSS::string_from_property_id(property.property_id), property.value->to_string());
+        if (property.important == CSS::Important::Yes)
+            builder.append(" \033[31;1m!important\033[0m"sv);
+        builder.append('\n');
+    }
+    for (auto& property : declaration.custom_properties()) {
+        indent(builder, indent_levels);
+        builder.appendff("  {}: '{}'", property.key, property.value.value->to_string());
+        if (property.value.important == CSS::Important::Yes)
+            builder.append(" \033[31;1m!important\033[0m"sv);
+        builder.append('\n');
+    }
+}
+
 void dump_style_rule(StringBuilder& builder, CSS::CSSStyleRule const& rule, int indent_levels)
 {
     for (auto& selector : rule.selectors()) {
         dump_selector(builder, selector, indent_levels + 1);
     }
-    indent(builder, indent_levels);
-    builder.appendff("  Declarations ({}):\n", rule.declaration().length());
-    for (auto& property : rule.declaration().properties()) {
-        indent(builder, indent_levels);
-        builder.appendff("    {}: '{}'", CSS::string_from_property_id(property.property_id), property.value->to_string());
-        if (property.important == CSS::Important::Yes)
-            builder.append(" \033[31;1m!important\033[0m"sv);
-        builder.append('\n');
-    }
-    for (auto& property : rule.declaration().custom_properties()) {
-        indent(builder, indent_levels);
-        builder.appendff("    {}: '{}'", property.key, property.value.value->to_string());
-        if (property.value.important == CSS::Important::Yes)
-            builder.append(" \033[31;1m!important\033[0m"sv);
-        builder.append('\n');
-    }
+    dump_declaration(builder, rule.declaration(), indent_levels + 1);
 }
 
 void dump_sheet(CSS::StyleSheet const& sheet)
@@ -883,4 +892,11 @@ void dump_namespace_rule(StringBuilder& builder, CSS::CSSNamespaceRule const& na
         builder.appendff("  Prefix: {}\n", namespace_.prefix());
 }
 
+void dump_nested_declarations(StringBuilder& builder, CSS::CSSNestedDeclarations const& declarations, int indent_levels)
+{
+    indent(builder, indent_levels);
+    builder.append("  Nested declarations:\n"sv);
+    dump_declaration(builder, declarations.declaration(), indent_levels + 1);
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/Dump.h b/Userland/Libraries/LibWeb/Dump.h
index a138f86fa54..3fa875261e8 100644
--- a/Userland/Libraries/LibWeb/Dump.h
+++ b/Userland/Libraries/LibWeb/Dump.h
@@ -24,12 +24,14 @@ void dump_sheet(StringBuilder&, CSS::StyleSheet const&);
 void dump_sheet(CSS::StyleSheet const&);
 void dump_rule(StringBuilder&, CSS::CSSRule const&, int indent_levels = 0);
 void dump_rule(CSS::CSSRule const&);
+void dump_declaration(StringBuilder&, CSS::PropertyOwningCSSStyleDeclaration const&, int indent_levels = 0);
 void dump_font_face_rule(StringBuilder&, CSS::CSSFontFaceRule const&, int indent_levels = 0);
 void dump_import_rule(StringBuilder&, CSS::CSSImportRule const&, int indent_levels = 0);
 void dump_media_rule(StringBuilder&, CSS::CSSMediaRule const&, int indent_levels = 0);
 void dump_style_rule(StringBuilder&, CSS::CSSStyleRule const&, int indent_levels = 0);
 void dump_supports_rule(StringBuilder&, CSS::CSSSupportsRule const&, int indent_levels = 0);
 void dump_namespace_rule(StringBuilder&, CSS::CSSNamespaceRule const&, int indent_levels = 0);
+void dump_nested_declarations(StringBuilder&, CSS::CSSNestedDeclarations const&, int indent_levels = 0);
 void dump_layer_block_rule(StringBuilder&, CSS::CSSLayerBlockRule const&, int indent_levels = 0);
 void dump_layer_statement_rule(StringBuilder&, CSS::CSSLayerStatementRule const&, int indent_levels = 0);
 void dump_selector(StringBuilder&, CSS::Selector const&, int indent_levels = 0);
diff --git a/Userland/Libraries/LibWeb/Forward.h b/Userland/Libraries/LibWeb/Forward.h
index ab5075a13c2..4c9bc2e0ec4 100644
--- a/Userland/Libraries/LibWeb/Forward.h
+++ b/Userland/Libraries/LibWeb/Forward.h
@@ -117,6 +117,7 @@ class CSSLayerBlockRule;
 class CSSLayerStatementRule;
 class CSSMathValue;
 class CSSMediaRule;
+class CSSNestedDeclarations;
 class CSSOKLab;
 class CSSOKLCH;
 class CSSRGB;
diff --git a/Userland/Libraries/LibWeb/idl_files.cmake b/Userland/Libraries/LibWeb/idl_files.cmake
index 19f61e53f2a..7e0abf596ff 100644
--- a/Userland/Libraries/LibWeb/idl_files.cmake
+++ b/Userland/Libraries/LibWeb/idl_files.cmake
@@ -25,6 +25,7 @@ libweb_js_bindings(CSS/CSSLayerStatementRule)
 libweb_js_bindings(CSS/CSSMediaRule)
 libweb_js_bindings(CSS/CSS NAMESPACE)
 libweb_js_bindings(CSS/CSSNamespaceRule)
+libweb_js_bindings(CSS/CSSNestedDeclarations)
 libweb_js_bindings(CSS/CSSRule)
 libweb_js_bindings(CSS/CSSRuleList)
 libweb_js_bindings(CSS/CSSStyleDeclaration)