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diff --git a/servo/components/style/stylesheets/margin_rule.rs b/servo/components/style/stylesheets/margin_rule.rs
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+++ b/servo/components/style/stylesheets/margin_rule.rs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! A [`@margin`][margin] rule.
+//!
+//! [margin]: https://drafts.csswg.org/css-page-3/#margin-boxes
+
+use crate::properties::PropertyDeclarationBlock;
+use crate::shared_lock::{DeepCloneParams, DeepCloneWithLock, Locked};
+use crate::shared_lock::{SharedRwLock, SharedRwLockReadGuard, ToCssWithGuard};
+use crate::str::CssStringWriter;
+use cssparser::SourceLocation;
+#[cfg(feature = "gecko")]
+use malloc_size_of::{MallocSizeOf, MallocSizeOfOps, MallocUnconditionalShallowSizeOf};
+use servo_arc::Arc;
+use std::fmt::{self, Write};
+
+macro_rules! margin_rule_types {
+    ($($(#[$($meta:tt)+])* $id:ident => $val:literal,)+) => {
+        /// [`@margin`][margin] rule names.
+        ///
+        /// https://drafts.csswg.org/css-page-3/#margin-at-rules
+        #[derive(Clone, Copy, Debug, Eq, MallocSizeOf, PartialEq, ToShmem)]
+        #[repr(u8)]
+        pub enum MarginRuleType {
+            $($(#[$($meta)+])* $id,)+
+        }
+
+        impl MarginRuleType {
+            #[inline]
+            fn to_str(&self) -> &'static str {
+                match *self {
+                    $(MarginRuleType::$id => concat!('@', $val),)+
+                }
+            }
+            /// Matches the rule type for this name. This does not expect a
+            /// leading '@'.
+            pub fn match_name(name: &str) -> Option<Self> {
+                Some(match_ignore_ascii_case! { name,
+                    $( $val => MarginRuleType::$id, )+
+                    _ => return None,
+                })
+            }
+        }
+    }
+}
+
+margin_rule_types! {
+    /// [`@top-left-corner`][top-left-corner] margin rule
+    ///
+    /// [top-left-corner] https://drafts.csswg.org/css-page-3/#top-left-corner-box-def
+    TopLeftCorner => "top-left-corner",
+    /// [`@top-left`][top-left] margin rule
+    ///
+    /// [top-left] https://drafts.csswg.org/css-page-3/#top-left-box-def
+    TopLeft => "top-left",
+    /// [`@top-center`][top-center] margin rule
+    ///
+    /// [top-center] https://drafts.csswg.org/css-page-3/#top-center-box-def
+    TopCenter => "top-center",
+    /// [`@top-right`][top-right] margin rule
+    ///
+    /// [top-right] https://drafts.csswg.org/css-page-3/#top-right-box-def
+    TopRight => "top-right",
+    /// [`@top-right-corner`][top-right-corner] margin rule
+    ///
+    /// [top-right-corner] https://drafts.csswg.org/css-page-3/#top-right-corner-box-def
+    TopRightCorner => "top-right-corner",
+    /// [`@bottom-left-corner`][bottom-left-corner] margin rule
+    ///
+    /// [bottom-left-corner] https://drafts.csswg.org/css-page-3/#bottom-left-corner-box-def
+    BottomLeftCorner => "bottom-left-corner",
+    /// [`@bottom-left`][bottom-left] margin rule
+    ///
+    /// [bottom-left] https://drafts.csswg.org/css-page-3/#bottom-left-box-def
+    BottomLeft => "bottom-left",
+    /// [`@bottom-center`][bottom-center] margin rule
+    ///
+    /// [bottom-center] https://drafts.csswg.org/css-page-3/#bottom-center-box-def
+    BottomCenter => "bottom-center",
+    /// [`@bottom-right`][bottom-right] margin rule
+    ///
+    /// [bottom-right] https://drafts.csswg.org/css-page-3/#bottom-right-box-def
+    BottomRight => "bottom-right",
+    /// [`@bottom-right-corner`][bottom-right-corner] margin rule
+    ///
+    /// [bottom-right-corner] https://drafts.csswg.org/css-page-3/#bottom-right-corner-box-def
+    BottomRightCorner => "bottom-right-corner",
+    /// [`@left-top`][left-top] margin rule
+    ///
+    /// [left-top] https://drafts.csswg.org/css-page-3/#left-top-box-def
+    LeftTop => "left-top",
+    /// [`@left-middle`][left-middle] margin rule
+    ///
+    /// [left-middle] https://drafts.csswg.org/css-page-3/#left-middle-box-def
+    LeftMiddle => "left-middle",
+    /// [`@left-bottom`][left-bottom] margin rule
+    ///
+    /// [left-bottom] https://drafts.csswg.org/css-page-3/#left-bottom-box-def
+    LeftBottom => "left-bottom",
+    /// [`@right-top`][right-top] margin rule
+    ///
+    /// [right-top] https://drafts.csswg.org/css-page-3/#right-top-box-def
+    RightTop => "right-top",
+    /// [`@right-middle`][right-middle] margin rule
+    ///
+    /// [right-middle] https://drafts.csswg.org/css-page-3/#right-middle-box-def
+    RightMiddle => "right-middle",
+    /// [`@right-bottom`][right-bottom] margin rule
+    ///
+    /// [right-bottom] https://drafts.csswg.org/css-page-3/#right-bottom-box-def
+    RightBottom => "right-bottom",
+}
+
+/// A [`@margin`][margin] rule.
+///
+/// [margin]: https://drafts.csswg.org/css-page-3/#margin-at-rules
+#[derive(Clone, Debug, ToShmem)]
+pub struct MarginRule {
+    /// Type of this margin rule.
+    pub rule_type: MarginRuleType,
+    /// The declaration block this margin rule contains.
+    pub block: Arc<Locked<PropertyDeclarationBlock>>,
+    /// The source position this rule was found at.
+    pub source_location: SourceLocation,
+}
+
+impl MarginRule {
+    /// Measure heap usage.
+    #[cfg(feature = "gecko")]
+    pub fn size_of(&self, guard: &SharedRwLockReadGuard, ops: &mut MallocSizeOfOps) -> usize {
+        // Measurement of other fields may be added later.
+        self.block.unconditional_shallow_size_of(ops) +
+            self.block.read_with(guard).size_of(ops)
+    }
+}
+
+impl ToCssWithGuard for MarginRule {
+    /// Serialization of a margin-rule is not specced, this is adapted from how
+    /// page-rules and style-rules are serialized.
+    fn to_css(&self, guard: &SharedRwLockReadGuard, dest: &mut CssStringWriter) -> fmt::Result {
+        dest.write_str(self.rule_type.to_str())?;
+        dest.write_str(" { ")?;
+        let declaration_block = self.block.read_with(guard);
+        declaration_block.to_css(dest)?;
+        if !declaration_block.declarations().is_empty() {
+            dest.write_char(' ')?;
+        }
+        dest.write_char('}')
+    }
+}
+
+impl DeepCloneWithLock for MarginRule {
+    fn deep_clone_with_lock(
+        &self,
+        lock: &SharedRwLock,
+        guard: &SharedRwLockReadGuard,
+        _params: &DeepCloneParams,
+    ) -> Self {
+        MarginRule {
+            rule_type: self.rule_type,
+            block: Arc::new(lock.wrap(self.block.read_with(&guard).clone())),
+            source_location: self.source_location.clone(),
+        }
+    }
+}