2 files changed, 22 insertions, 17 deletions

diff --git a/src/components/main/layout/box_.rs b/src/components/main/layout/box_.rs
index 19a9ed9adba..f5acd633944 100644
--- a/src/components/main/layout/box_.rs
+++ b/src/components/main/layout/box_.rs
@@ -617,19 +617,19 @@ impl Box {
         let style = self.style();
         let width = MaybeAuto::from_style(style.Box.get().width, Au::new(0)).specified_or_zero();
 
-        let (mut margin_left, mut margin_right) = (Au(0), Au(0));
-        if use_margins {
-            margin_left = MaybeAuto::from_style(style.Margin.get().margin_left,
-                                                Au(0)).specified_or_zero();
-            margin_right = MaybeAuto::from_style(style.Margin.get().margin_right,
-                                                 Au(0)).specified_or_zero();
-        }
+        let (margin_left, margin_right) = if use_margins {
+            (MaybeAuto::from_style(style.Margin.get().margin_left, Au(0)).specified_or_zero(),
+             MaybeAuto::from_style(style.Margin.get().margin_right, Au(0)).specified_or_zero())
+        } else {
+            (Au(0), Au(0))
+        };
 
-        let (mut padding_left, mut padding_right) = (Au(0), Au(0));
-        if use_padding {
-            padding_left = self.compute_padding_length(style.Padding.get().padding_left, Au(0));
-            padding_right = self.compute_padding_length(style.Padding.get().padding_right, Au(0));
-        }
+        let (padding_left, padding_right) = if use_padding {
+            (self.compute_padding_length(style.Padding.get().padding_left, Au(0)),
+             self.compute_padding_length(style.Padding.get().padding_right, Au(0)))
+        } else {
+            (Au(0), Au(0))
+        };
 
         let surround_width = margin_left + margin_right + padding_left + padding_right +
                 self.border.get().left + self.border.get().right;
diff --git a/src/components/main/layout/model.rs b/src/components/main/layout/model.rs
index 8dacb8c6ea4..378c43d1bb6 100644
--- a/src/components/main/layout/model.rs
+++ b/src/components/main/layout/model.rs
@@ -7,6 +7,7 @@
 use layout::box_::Box;
 
 use computed = style::computed_values;
+use style::computed_values::{LPA_Auto, LPA_Length, LPA_Percentage};
 use servo_util::geometry::Au;
 use servo_util::geometry;
 
@@ -61,7 +62,8 @@ pub enum CollapsibleMargins {
     /// margins do not collapse through this flow.
     MarginsCollapse(AdjoiningMargins, AdjoiningMargins),
 
-    /// Margins collapse *through* this flow. This means, essentially, that the flow is empty.
+    /// Margins collapse *through* this flow. This means, essentially, that the flow doesn’t
+    /// have any border, padding, or out-of-flow (floating or positioned) content
     MarginsCollapseThrough(AdjoiningMargins),
 }
 
@@ -108,9 +110,11 @@ impl MarginCollapseInfo {
                                                   -> (CollapsibleMargins, Au) {
         let state = match self.state {
             AccumulatingCollapsibleTopMargin => {
-                match MaybeAuto::from_style(fragment.style().Box.get().height, Au(0)) {
-                    Auto | Specified(Au(0)) => MarginsCollapseThroughFinalMarginState,
-                    Specified(_) => {
+                match fragment.style().Box.get().height {
+                    LPA_Auto | LPA_Length(Au(0)) | LPA_Percentage(0.) => {
+                        MarginsCollapseThroughFinalMarginState
+                    },
+                    _ => {
                         // If the box has an explicitly specified height, margins may not collapse
                         // through it.
                         BottomMarginCollapsesFinalMarginState
@@ -205,7 +209,8 @@ impl MarginCollapseInfo {
                 Au(0)
             }
             (AccumulatingMarginIn, NoCollapsibleMargins(_, bottom)) => {
-                // Margin-in should have been set to zero above.
+                assert_eq!(self.margin_in.most_positive, Au(0));
+                assert_eq!(self.margin_in.most_negative, Au(0));
                 bottom
             }
             (AccumulatingMarginIn, MarginsCollapse(_, bottom)) |