diff --git a/packages/react-native/ReactCommon/react/renderer/css/CSSDeclaredStyle.h b/packages/react-native/ReactCommon/react/renderer/css/CSSDeclaredStyle.h
new file mode 100644
index 00000000000..c52bf22e679
--- /dev/null
+++ b/packages/react-native/ReactCommon/react/renderer/css/CSSDeclaredStyle.h
@@ -0,0 +1,108 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <algorithm>
+#include <bitset>
+#include <memory>
+#include <type_traits>
+#include <vector>
+
+#include <react/debug/react_native_assert.h>
+#include <react/renderer/css/CSSParser.h>
+#include <react/renderer/css/CSSProperties.h>
+
+namespace facebook::react {
+
+/**
+ * CSSDeclaredStyle represents the set of style declarations on an element set
+ * by the user. Users should generally not read from CSSDeclaredStyle directly,
+ * and should instead use the computed style calculated on ShadowTree commit.
+ */
+class CSSDeclaredStyle {
+ public:
+  template <CSSProp Prop>
+  void set(const CSSDeclaredValue<Prop>& value) {
+    using DeclaredValueT = std::remove_cvref_t<CSSDeclaredValue<Prop>>;
+    static_assert(sizeof(value) <= sizeof(PropMapping::value));
+    static_assert(std::is_trivially_destructible_v<DeclaredValueT>);
+
+    if (specifiedProperties_.test(to_underlying(Prop))) {
+      auto it = std::lower_bound(
+          properties_.begin(), properties_.end(), PropMapping{Prop, {}});
+      react_native_assert(it->prop == Prop);
+      std::construct_at(
+          reinterpret_cast<DeclaredValueT*>(it->value.data()), value);
+    } else {
+      auto it = std::upper_bound(
+          properties_.begin(), properties_.end(), PropMapping{Prop, {}});
+      it = properties_.insert(it, {Prop, {}});
+      std::construct_at(
+          reinterpret_cast<DeclaredValueT*>(it->value.data()), value);
+      specifiedProperties_.set(to_underlying(Prop));
+    }
+  }
+
+  template <CSSProp Prop>
+  void set(std::string_view value) {
+    set<Prop>(parseCSSProp<Prop>(value));
+  }
+
+  /**
+   * Returns the declared value, represented as the "unset" keyword if never
+   * specified. Additional shorthands can be provided in order
+   * of precedence if Prop is unset.
+   */
+  template <CSSProp Prop, CSSProp... ShorthandsT>
+  CSSDeclaredValue<Prop> get() const {
+    if (specifiedProperties_.test(to_underlying(Prop))) {
+      auto it = std::lower_bound(
+          properties_.begin(), properties_.end(), PropMapping{Prop, {}});
+      react_native_assert(it->prop == Prop);
+
+      CSSDeclaredValue<Prop> value{*std::launder(
+          reinterpret_cast<const CSSDeclaredValue<Prop>*>(it->value.data()))};
+
+      if (value) {
+        return value;
+      }
+    }
+
+    if constexpr (sizeof...(ShorthandsT) == 0) {
+      return {};
+    } else {
+      return get<ShorthandsT...>();
+    }
+  }
+
+  bool operator==(const CSSDeclaredStyle& rhs) const = default;
+
+ private:
+  struct PropMapping {
+    CSSProp prop;
+    std::array<
+        std::byte,
+        sizeof(CSSValueVariant<
+               CSSWideKeyword,
+               CSSKeyword,
+               CSSLength,
+               CSSNumber,
+               CSSPercentage,
+               CSSRatio>)>
+        value;
+
+    constexpr bool operator<(const PropMapping& rhs) const {
+      return to_underlying(prop) < to_underlying(rhs.prop);
+    }
+  };
+
+  std::vector<PropMapping> properties_;
+  std::bitset<kCSSPropCount> specifiedProperties_;
+};
+
+} // namespace facebook::react
diff --git a/packages/react-native/ReactCommon/react/renderer/css/CSSParser.h b/packages/react-native/ReactCommon/react/renderer/css/CSSParser.h
index f9ad61c4846..c2ea7d9bc94 100644
--- a/packages/react-native/ReactCommon/react/renderer/css/CSSParser.h
+++ b/packages/react-native/ReactCommon/react/renderer/css/CSSParser.h
@@ -212,7 +212,7 @@ CSSValueVariant<AllowedTypesT...> parseCSSComponentValue(std::string_view css) {
 template <CSSProp Prop>
 constexpr auto parseCSSProp(std::string_view css) {
   // For now we only allow parsing props composed of a single component value.
-  CSSSpecifiedValue<Prop> value;
+  CSSDeclaredValue<Prop> value;
   parseCSSComponentValue(css, value);
   return value;
 }
diff --git a/packages/react-native/ReactCommon/react/renderer/css/CSSProperties.h b/packages/react-native/ReactCommon/react/renderer/css/CSSProperties.h
index 067d6c2b3a1..582dbb4385c 100644
--- a/packages/react-native/ReactCommon/react/renderer/css/CSSProperties.h
+++ b/packages/react-native/ReactCommon/react/renderer/css/CSSProperties.h
@@ -137,16 +137,22 @@ struct CSSPropDefinition {};
 template <CSSProp P>
 using CSSAllowedKeywords = typename CSSPropDefinition<P>::Keyword;
 
+template <CSSProp P>
+using CSSDeclaredValue = typename CSSPropDefinition<P>::DeclaredValue;
+
 template <CSSProp P>
 using CSSSpecifiedValue = typename CSSPropDefinition<P>::SpecifiedValue;
 
 template <CSSProp P>
 using CSSComputedValue = typename CSSPropDefinition<P>::ComputedValue;
 
-struct CSSComputeContext {
-  bool isRTL{false};
-  bool useWebDefaults{false};
-  bool swapLeftAndRight{false};
+/**
+ * Whether to behave in accordance with W3C specs, or to incorporate React
+ * Native specific tweaks to defaults and computation.
+ */
+enum class CSSFlavor {
+  W3C,
+  ReactNative,
 };
 
 /**
@@ -168,16 +174,18 @@ struct CSSPropDefinition<CSSProp::AlignContent> {
     End = to_underlying(CSSKeyword::End),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext& ctx) {
-    return ctx.useWebDefaults ? SpecifiedValue::keyword(Keyword::Stretch)
-                              : SpecifiedValue::keyword(Keyword::FlexStart);
+  constexpr static DeclaredValue initialValue(CSSFlavor flavor) {
+    return flavor == CSSFlavor::W3C
+        ? DeclaredValue::keyword(Keyword::Stretch)
+        : DeclaredValue::keyword(Keyword::FlexStart);
   }
 };
 
@@ -198,15 +206,16 @@ struct CSSPropDefinition<CSSProp::AlignItems> {
     End = to_underlying(CSSKeyword::End),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::Stretch);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::Stretch);
   }
 };
 
@@ -228,15 +237,16 @@ struct CSSPropDefinition<CSSProp::AlignSelf> {
     End = to_underlying(CSSKeyword::End),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::Auto);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::Auto);
   }
 };
 
@@ -250,15 +260,16 @@ struct CSSPropDefinition<CSSProp::AspectRatio> {
     Auto = to_underlying(CSSKeyword::Auto),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword, CSSRatio>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword, CSSRatio>;
+  using SpecifiedValue = CSSValueVariant<Keyword, CSSRatio>;
   using ComputedValue = CSSValueVariant<Keyword, CSSRatio>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::Auto);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::Auto);
   }
 };
 
@@ -268,16 +279,17 @@ struct CSSPropDefinition<CSSProp::AspectRatio> {
  */
 template <>
 struct CSSPropDefinition<CSSProp::BorderRadius> {
-  using SpecifiedValue =
+  using DeclaredValue =
       CSSValueVariant<CSSWideKeyword, CSSLength, CSSPercentage>;
+  using SpecifiedValue = CSSValueVariant<CSSLength, CSSPercentage>;
   using ComputedValue = CSSValueVariant<CSSLength, CSSPercentage>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::length(0.0f, CSSLengthUnit::Px);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::length(0.0f, CSSLengthUnit::Px);
   }
 };
 
@@ -332,16 +344,17 @@ struct CSSPropDefinition<CSSProp::BorderStyle> {
     Outset = to_underlying(CSSKeyword::Outset),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext& ctx) {
-    return ctx.useWebDefaults ? SpecifiedValue::keyword(Keyword::None)
-                              : SpecifiedValue::keyword(Keyword::Solid);
+  constexpr static DeclaredValue initialValue(CSSFlavor flavor) {
+    return flavor == CSSFlavor::W3C ? DeclaredValue::keyword(Keyword::None)
+                                    : DeclaredValue::keyword(Keyword::Solid);
   }
 };
 
@@ -397,16 +410,18 @@ struct CSSPropDefinition<CSSProp::BorderWidth> {
     Thick = to_underlying(CSSKeyword::Thick),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword, CSSLength>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword, CSSLength>;
+  using SpecifiedValue = CSSValueVariant<CSSLength, CSSKeyword>;
   using ComputedValue = CSSValueVariant<CSSLength>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext& ctx) {
-    return ctx.useWebDefaults ? SpecifiedValue::keyword(Keyword::Medium)
-                              : SpecifiedValue::length(0.0f, CSSLengthUnit::Px);
+  constexpr static DeclaredValue initialValue(CSSFlavor flavor) {
+    return flavor == CSSFlavor::W3C
+        ? DeclaredValue::keyword(Keyword::Medium)
+        : DeclaredValue::length(0.0f, CSSLengthUnit::Px);
   }
 };
 
@@ -477,15 +492,16 @@ struct CSSPropDefinition<CSSProp::Direction> {
     Rtl = to_underlying(CSSKeyword::Rtl),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return true;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::Ltr);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::Ltr);
   }
 };
 
@@ -507,16 +523,17 @@ struct CSSPropDefinition<CSSProp::Display> {
     InlineGrid = to_underlying(CSSKeyword::InlineGrid),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext& ctx) {
-    return ctx.useWebDefaults ? SpecifiedValue::keyword(Keyword::Inline)
-                              : SpecifiedValue::keyword(Keyword::Flex);
+  constexpr static DeclaredValue initialValue(CSSFlavor flavor) {
+    return flavor == CSSFlavor::W3C ? DeclaredValue::keyword(Keyword::Inline)
+                                    : DeclaredValue::keyword(Keyword::Flex);
   }
 };
 
@@ -534,15 +551,16 @@ struct CSSPropDefinition<CSSProp::Flex> {
     None = to_underlying(CSSKeyword::None),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword, CSSNumber>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword, CSSNumber>;
+  using SpecifiedValue = CSSValueVariant<Keyword, CSSNumber>;
   using ComputedValue = CSSValueVariant<Keyword, CSSNumber>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::number(0.0f);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::number(0.0f);
   }
 };
 
@@ -557,16 +575,17 @@ struct CSSPropDefinition<CSSProp::FlexBasis> {
     Content = to_underlying(CSSKeyword::Content),
   };
 
-  using SpecifiedValue =
+  using DeclaredValue =
       CSSValueVariant<CSSWideKeyword, Keyword, CSSLength, CSSPercentage>;
+  using SpecifiedValue = CSSValueVariant<Keyword, CSSLength, CSSPercentage>;
   using ComputedValue = CSSValueVariant<Keyword, CSSLength, CSSPercentage>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::Auto);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::Auto);
   }
 };
 
@@ -583,16 +602,17 @@ struct CSSPropDefinition<CSSProp::FlexDirection> {
     ColumnReverse = to_underlying(CSSKeyword::ColumnReverse),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext& ctx) {
-    return ctx.useWebDefaults ? SpecifiedValue::keyword(Keyword::Row)
-                              : SpecifiedValue::keyword(Keyword::Column);
+  constexpr static DeclaredValue initialValue(CSSFlavor flavor) {
+    return flavor == CSSFlavor::W3C ? DeclaredValue::keyword(Keyword::Row)
+                                    : DeclaredValue::keyword(Keyword::Column);
   }
 };
 
@@ -602,15 +622,16 @@ struct CSSPropDefinition<CSSProp::FlexDirection> {
  */
 template <>
 struct CSSPropDefinition<CSSProp::FlexGrow> {
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, CSSNumber>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, CSSNumber>;
+  using SpecifiedValue = CSSValueVariant<CSSNumber>;
   using ComputedValue = CSSValueVariant<CSSNumber>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::number(0.0f);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::number(0.0f);
   }
 };
 
@@ -620,16 +641,17 @@ struct CSSPropDefinition<CSSProp::FlexGrow> {
  */
 template <>
 struct CSSPropDefinition<CSSProp::FlexShrink> {
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, CSSNumber>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, CSSNumber>;
+  using SpecifiedValue = CSSValueVariant<CSSNumber>;
   using ComputedValue = CSSValueVariant<CSSNumber>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext& ctx) {
-    return ctx.useWebDefaults ? SpecifiedValue::number(1.0f)
-                              : SpecifiedValue::number(0.0f);
+  constexpr static DeclaredValue initialValue(CSSFlavor flavor) {
+    return flavor == CSSFlavor::W3C ? DeclaredValue::number(1.0f)
+                                    : DeclaredValue::number(0.0f);
   }
 };
 
@@ -645,15 +667,16 @@ struct CSSPropDefinition<CSSProp::FlexWrap> {
     WrapReverse = to_underlying(CSSKeyword::WrapReverse),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::NoWrap);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::NoWrap);
   }
 };
 
@@ -667,15 +690,16 @@ struct CSSPropDefinition<CSSProp::Gap> {
     Normal = to_underlying(CSSKeyword::Normal),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword, CSSLength>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword, CSSLength>;
+  using SpecifiedValue = CSSValueVariant<Keyword, CSSLength>;
   using ComputedValue = CSSValueVariant<Keyword, CSSLength>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::Normal);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::Normal);
   }
 };
 
@@ -698,16 +722,17 @@ struct CSSPropDefinition<CSSProp::Height> {
     MinContent = to_underlying(CSSKeyword::MinContent),
   };
 
-  using SpecifiedValue =
+  using DeclaredValue =
       CSSValueVariant<CSSWideKeyword, Keyword, CSSLength, CSSPercentage>;
+  using SpecifiedValue = CSSValueVariant<Keyword, CSSLength, CSSPercentage>;
   using ComputedValue = CSSValueVariant<Keyword, CSSLength, CSSPercentage>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::Auto);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::Auto);
   }
 };
 
@@ -741,16 +766,17 @@ struct CSSPropDefinition<CSSProp::Inset> {
     Auto = to_underlying(CSSKeyword::Auto),
   };
 
-  using SpecifiedValue =
+  using DeclaredValue =
       CSSValueVariant<CSSWideKeyword, Keyword, CSSLength, CSSPercentage>;
+  using SpecifiedValue = CSSValueVariant<Keyword, CSSLength, CSSPercentage>;
   using ComputedValue = CSSValueVariant<Keyword, CSSLength, CSSPercentage>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::Auto);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::Auto);
   }
 };
 
@@ -815,15 +841,16 @@ struct CSSPropDefinition<CSSProp::JustifyContent> {
     End = to_underlying(CSSKeyword::End),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::FlexStart);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::FlexStart);
   }
 };
 
@@ -837,16 +864,17 @@ struct CSSPropDefinition<CSSProp::Margin> {
     Auto = to_underlying(CSSKeyword::Auto),
   };
 
-  using SpecifiedValue =
+  using DeclaredValue =
       CSSValueVariant<CSSWideKeyword, Keyword, CSSLength, CSSPercentage>;
+  using SpecifiedValue = CSSValueVariant<Keyword, CSSLength, CSSPercentage>;
   using ComputedValue = CSSValueVariant<Keyword, CSSLength, CSSPercentage>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::length(0.0f, CSSLengthUnit::Px);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::length(0.0f, CSSLengthUnit::Px);
   }
 };
 
@@ -912,15 +940,16 @@ struct CSSPropDefinition<CSSProp::MarginVertical>
  */
 template <>
 struct CSSPropDefinition<CSSProp::Opacity> {
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, CSSNumber>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, CSSNumber>;
+  using SpecifiedValue = CSSValueVariant<CSSNumber>;
   using ComputedValue = CSSValueVariant<CSSNumber>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::number(1.0f);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::number(1.0f);
   }
 };
 
@@ -938,15 +967,16 @@ struct CSSPropDefinition<CSSProp::Overflow> {
     Visible = to_underlying(CSSKeyword::Visible),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::keyword(Keyword::Visible);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::keyword(Keyword::Visible);
   }
 };
 
@@ -956,16 +986,17 @@ struct CSSPropDefinition<CSSProp::Overflow> {
  */
 template <>
 struct CSSPropDefinition<CSSProp::Padding> {
-  using SpecifiedValue =
+  using DeclaredValue =
       CSSValueVariant<CSSWideKeyword, CSSLength, CSSPercentage>;
+  using SpecifiedValue = CSSValueVariant<CSSLength, CSSPercentage>;
   using ComputedValue = CSSValueVariant<CSSLength, CSSPercentage>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext&) {
-    return SpecifiedValue::length(0.0f, CSSLengthUnit::Px);
+  constexpr static DeclaredValue initialValue(CSSFlavor /*flavor*/) {
+    return DeclaredValue::length(0.0f, CSSLengthUnit::Px);
   }
 };
 
@@ -1039,16 +1070,17 @@ struct CSSPropDefinition<CSSProp::Position> {
     Sticky = to_underlying(CSSKeyword::Sticky),
   };
 
-  using SpecifiedValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using DeclaredValue = CSSValueVariant<CSSWideKeyword, Keyword>;
+  using SpecifiedValue = CSSValueVariant<Keyword>;
   using ComputedValue = CSSValueVariant<Keyword>;
 
   constexpr static bool isInherited() {
     return false;
   }
 
-  constexpr static SpecifiedValue initialValue(const CSSComputeContext& ctx) {
-    return ctx.useWebDefaults ? SpecifiedValue::keyword(Keyword::Static)
-                              : SpecifiedValue::keyword(Keyword::Relative);
+  constexpr static DeclaredValue initialValue(CSSFlavor flavor) {
+    return flavor == CSSFlavor::W3C ? DeclaredValue::keyword(Keyword::Static)
+                                    : DeclaredValue::keyword(Keyword::Relative);
   }
 };
 
diff --git a/packages/react-native/ReactCommon/react/renderer/css/CSSValue.h b/packages/react-native/ReactCommon/react/renderer/css/CSSValue.h
index 5d7d9b1ee41..afe7d436c43 100644
--- a/packages/react-native/ReactCommon/react/renderer/css/CSSValue.h
+++ b/packages/react-native/ReactCommon/react/renderer/css/CSSValue.h
@@ -36,9 +36,7 @@ enum class CSSValueType : uint8_t {
  */
 template <typename T>
 concept CSSDataType = std::is_trivially_destructible_v<T> &&
-    std::is_default_constructible_v<T> && requires() {
-  sizeof(T);
-};
+    std::is_copy_constructible_v<T> && std::is_default_constructible_v<T>;
 
 #pragma pack(push, 1)
 /**
diff --git a/packages/react-native/ReactCommon/react/renderer/css/tests/CSSDeclaredStyleTest.cpp b/packages/react-native/ReactCommon/react/renderer/css/tests/CSSDeclaredStyleTest.cpp
new file mode 100644
index 00000000000..caee42e5653
--- /dev/null
+++ b/packages/react-native/ReactCommon/react/renderer/css/tests/CSSDeclaredStyleTest.cpp
@@ -0,0 +1,151 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <gtest/gtest.h>
+#include <react/renderer/css/CSSDeclaredStyle.h>
+#include <react/renderer/css/CSSParser.h>
+
+namespace facebook::react {
+
+TEST(CSSDeclaredStyle, unset_keyword) {
+  CSSDeclaredStyle style;
+  auto value = style.get<CSSProp::FlexDirection>();
+  EXPECT_EQ(value.type(), CSSValueType::CSSWideKeyword);
+  EXPECT_EQ(value.getCSSWideKeyword(), CSSWideKeyword::Unset);
+}
+
+TEST(CSSDeclaredStyle, unset_ratio) {
+  CSSDeclaredStyle style;
+  auto value = style.get<CSSProp::AspectRatio>();
+  EXPECT_EQ(value.type(), CSSValueType::CSSWideKeyword);
+  EXPECT_EQ(value.getCSSWideKeyword(), CSSWideKeyword::Unset);
+}
+
+TEST(CSSDeclaredStyle, set_keyword) {
+  CSSDeclaredStyle style;
+
+  style.set<CSSProp::FlexDirection>("row");
+
+  auto value = style.get<CSSProp::FlexDirection>();
+  EXPECT_EQ(value.type(), CSSValueType::Keyword);
+  EXPECT_EQ(
+      value.getKeyword(), CSSAllowedKeywords<CSSProp::FlexDirection>::Row);
+}
+
+TEST(CSSDeclaredStyle, set_ratio) {
+  CSSDeclaredStyle style;
+
+  style.set<CSSProp::AspectRatio>("16 / 9");
+
+  auto value = style.get<CSSProp::AspectRatio>();
+  EXPECT_EQ(value.type(), CSSValueType::Ratio);
+  EXPECT_EQ(value.getRatio().numerator, 16.0f);
+  EXPECT_EQ(value.getRatio().denominator, 9.0f);
+}
+
+TEST(CSSDeclaredStyle, overwrite_ratio) {
+  CSSDeclaredStyle style;
+
+  style.set<CSSProp::AspectRatio>("16 / 9");
+
+  auto value1 = style.get<CSSProp::AspectRatio>();
+  EXPECT_EQ(value1.type(), CSSValueType::Ratio);
+  EXPECT_EQ(value1.getRatio().numerator, 16.0f);
+  EXPECT_EQ(value1.getRatio().denominator, 9.0f);
+
+  style.set<CSSProp::AspectRatio>("4/3");
+
+  auto value2 = style.get<CSSProp::AspectRatio>();
+  EXPECT_EQ(value2.type(), CSSValueType::Ratio);
+  EXPECT_EQ(value2.getRatio().numerator, 4.0f);
+  EXPECT_EQ(value2.getRatio().denominator, 3.0f);
+}
+
+TEST(CSSDeclaredStyle, set_multiple) {
+  CSSDeclaredStyle style;
+
+  style.set<CSSProp::FlexDirection>("row");
+  style.set<CSSProp::AspectRatio>("16 / 9");
+
+  auto flexDirection = style.get<CSSProp::FlexDirection>();
+  EXPECT_EQ(flexDirection.type(), CSSValueType::Keyword);
+  EXPECT_EQ(
+      flexDirection.getKeyword(),
+      CSSAllowedKeywords<CSSProp::FlexDirection>::Row);
+
+  auto aspectRatio = style.get<CSSProp::AspectRatio>();
+  EXPECT_EQ(aspectRatio.type(), CSSValueType::Ratio);
+  EXPECT_EQ(aspectRatio.getRatio().numerator, 16.0f);
+  EXPECT_EQ(aspectRatio.getRatio().denominator, 9.0f);
+}
+
+TEST(CSSDeclaredStyle, set_multiple_overwrite) {
+  CSSDeclaredStyle style;
+
+  style.set<CSSProp::FlexDirection>("row");
+  style.set<CSSProp::AspectRatio>("16 / 9");
+  style.set<CSSProp::FlexDirection>("column");
+
+  auto flexDirection = style.get<CSSProp::FlexDirection>();
+  EXPECT_EQ(flexDirection.type(), CSSValueType::Keyword);
+  EXPECT_EQ(
+      flexDirection.getKeyword(),
+      CSSAllowedKeywords<CSSProp::FlexDirection>::Column);
+
+  auto aspectRatio = style.get<CSSProp::AspectRatio>();
+  EXPECT_EQ(aspectRatio.type(), CSSValueType::Ratio);
+  EXPECT_EQ(aspectRatio.getRatio().numerator, 16.0f);
+  EXPECT_EQ(aspectRatio.getRatio().denominator, 9.0f);
+}
+
+TEST(CSSDeclaredStyle, set_multiple_reset) {
+  CSSDeclaredStyle style;
+
+  style.set<CSSProp::FlexDirection>("row");
+  style.set<CSSProp::AspectRatio>("16 / 9");
+  style.set<CSSProp::FlexDirection>("");
+
+  auto flexDirection = style.get<CSSProp::FlexDirection>();
+  EXPECT_EQ(flexDirection.type(), CSSValueType::CSSWideKeyword);
+  EXPECT_EQ(flexDirection.getCSSWideKeyword(), CSSWideKeyword::Unset);
+
+  auto aspectRatio = style.get<CSSProp::AspectRatio>();
+  EXPECT_EQ(aspectRatio.type(), CSSValueType::Ratio);
+  EXPECT_EQ(aspectRatio.getRatio().numerator, 16.0f);
+  EXPECT_EQ(aspectRatio.getRatio().denominator, 9.0f);
+}
+
+TEST(CSSDeclaredStyle, get_with_precedence) {
+  CSSDeclaredStyle style;
+
+  style.set<CSSProp::Margin>("1px");
+
+  auto margin1 = style.get<
+      CSSProp::MarginStart,
+      CSSProp::MarginLeft,
+      CSSProp::MarginInline,
+      CSSProp::MarginHorizontal,
+      CSSProp::Margin>();
+
+  EXPECT_EQ(margin1.type(), CSSValueType::Length);
+  EXPECT_EQ(margin1.getLength().value, 1.0f);
+  EXPECT_EQ(margin1.getLength().unit, CSSLengthUnit::Px);
+
+  style.set<CSSProp::MarginLeft>("3px");
+  auto margin2 = style.get<
+      CSSProp::MarginStart,
+      CSSProp::MarginLeft,
+      CSSProp::MarginInline,
+      CSSProp::MarginHorizontal,
+      CSSProp::Margin>();
+
+  EXPECT_EQ(margin2.type(), CSSValueType::Length);
+  EXPECT_EQ(margin2.getLength().value, 3.0f);
+  EXPECT_EQ(margin2.getLength().unit, CSSLengthUnit::Px);
+}
+
+} // namespace facebook::react