diff --git a/components/style/properties/properties.mako.rs b/components/style/properties/properties.mako.rs
index 78c3865b999..a865c20a598 100644
--- a/components/style/properties/properties.mako.rs
+++ b/components/style/properties/properties.mako.rs
@@ -17,9 +17,10 @@ use custom_properties::CustomPropertiesBuilder;
 use servo_arc::{Arc, UniqueArc};
 use smallbitvec::SmallBitVec;
 use std::borrow::Cow;
-use std::{mem, ops};
+use std::{ops, ptr};
 use std::cell::RefCell;
 use std::fmt::{self, Write};
+use std::mem::{self, ManuallyDrop};
 
 #[cfg(feature = "servo")] use cssparser::RGBA;
 use cssparser::{CowRcStr, Parser, TokenSerializationType, serialize_identifier};
@@ -1441,28 +1442,94 @@ impl PropertyId {
     }
 }
 
-/// Servo's representation for a property declaration.
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[derive(Clone, PartialEq)]
-pub enum PropertyDeclaration {
-    % for property in data.longhands:
-    <%
+<%
+    variants = []
+    for property in data.longhands:
         if property.predefined_type and not property.is_vector and not property.is_gecko_size_type_hack:
             ty = "::values::specified::{}".format(property.predefined_type)
         else:
             ty = "longhands::{}::SpecifiedValue".format(property.ident)
         if property.boxed:
             ty = "Box<{}>".format(ty)
-    %>
-    /// ${property.name}
-    ${property.camel_case}(${ty}),
+        variants.append({
+            "name": property.camel_case,
+            "type": ty,
+            "doc": "`" + property.name + "`",
+        })
+    variants += [
+        {
+            "name": "CSSWideKeyword",
+            "type": "WideKeywordDeclaration",
+            "doc": "A CSS-wide keyword.",
+        },
+        {
+            "name": "WithVariables",
+            "type": "VariableDeclaration",
+            "doc": "An unparsed declaration.",
+        },
+        {
+            "name": "Custom",
+            "type": "CustomDeclaration",
+            "doc": "A custom property declaration.",
+        },
+    ]
+%>
+
+/// Servo's representation for a property declaration.
+#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
+#[derive(PartialEq)]
+#[repr(u16)]
+pub enum PropertyDeclaration {
+    % for variant in variants:
+    /// ${variant["doc"]}
+    ${variant["name"]}(${variant["type"]}),
     % endfor
-    /// A css-wide keyword.
-    CSSWideKeyword(WideKeywordDeclaration),
-    /// An unparsed declaration.
-    WithVariables(VariableDeclaration),
-    /// A custom property declaration.
-    Custom(CustomDeclaration),
+}
+
+<%
+    from itertools import groupby
+
+    groups = {}
+    keyfunc = lambda x: x["type"]
+    for ty, group in groupby(sorted(variants, key=keyfunc), keyfunc):
+        groups[ty] = list(group)
+%>
+
+impl Clone for PropertyDeclaration {
+    #[inline]
+    fn clone(&self) -> Self {
+        use self::PropertyDeclaration::*;
+
+        #[repr(C)]
+        struct VariantRepr<T> {
+            tag: u16,
+            value: T
+        }
+
+        match *self {
+            % for ty, variants in groups.iteritems():
+            % if len(variants) == 1:
+            ${variants[0]["name"]}(ref value) => {
+                ${variants[0]["name"]}(value.clone())
+            }
+            % else:
+            ${" | ".join("{}(ref value)".format(v["name"]) for v in variants)} => {
+                unsafe {
+                    let mut out = ManuallyDrop::new(mem::uninitialized());
+                    ptr::write(
+                        &mut out as *mut _ as *mut VariantRepr<${ty}>,
+                        VariantRepr {
+                            tag: *(self as *const _ as *const u16),
+                            value: value.clone(),
+                        },
+                    );
+                    ManuallyDrop::into_inner(out)
+                }
+            }
+            % endif
+            % endfor
+        }
+    }
 }
 
 /// A declaration using a CSS-wide keyword.