diff --git a/components/layout/block.rs b/components/layout/block.rs
index 24eac4f48ea..236bfd321ea 100644
--- a/components/layout/block.rs
+++ b/components/layout/block.rs
@@ -1990,7 +1990,8 @@ pub struct ISizeConstraintSolution {
 }
 
 impl ISizeConstraintSolution {
-    pub fn new(inline_size: Au, margin_inline_start: Au, margin_inline_end: Au) -> ISizeConstraintSolution {
+    pub fn new(inline_size: Au, margin_inline_start: Au, margin_inline_end: Au)
+               -> ISizeConstraintSolution {
         ISizeConstraintSolution {
             inline_start: Au(0),
             inline_end: Au(0),
diff --git a/components/layout/model.rs b/components/layout/model.rs
index 64ba9a001d8..2479ab7cb47 100644
--- a/components/layout/model.rs
+++ b/components/layout/model.rs
@@ -174,7 +174,12 @@ impl MarginCollapseInfo {
 
     pub fn current_float_ceiling(&mut self) -> Au {
         match self.state {
-            MarginCollapseState::AccumulatingCollapsibleTopMargin => self.block_start_margin.collapse(),
+            MarginCollapseState::AccumulatingCollapsibleTopMargin => {
+                // We do not include the top margin in the float ceiling, because the float flow
+                // needs to be positioned relative to our *border box*, not our margin box. See
+                // `tests/ref/float_under_top_margin_a.html`.
+                Au(0)
+            }
             MarginCollapseState::AccumulatingMarginIn => self.margin_in.collapse(),
         }
     }
@@ -182,18 +187,22 @@ impl MarginCollapseInfo {
     /// Adds the child's potentially collapsible block-start margin to the current margin state and
     /// advances the Y offset by the appropriate amount to handle that margin. Returns the amount
     /// that should be added to the Y offset during block layout.
-    pub fn advance_block_start_margin(&mut self, child_collapsible_margins: &CollapsibleMargins) -> Au {
+    pub fn advance_block_start_margin(&mut self, child_collapsible_margins: &CollapsibleMargins)
+                                      -> Au {
         match (self.state, *child_collapsible_margins) {
-            (MarginCollapseState::AccumulatingCollapsibleTopMargin, CollapsibleMargins::None(block_start, _)) => {
+            (MarginCollapseState::AccumulatingCollapsibleTopMargin,
+             CollapsibleMargins::None(block_start, _)) => {
                 self.state = MarginCollapseState::AccumulatingMarginIn;
                 block_start
             }
-            (MarginCollapseState::AccumulatingCollapsibleTopMargin, CollapsibleMargins::Collapse(block_start, _)) => {
+            (MarginCollapseState::AccumulatingCollapsibleTopMargin,
+             CollapsibleMargins::Collapse(block_start, _)) => {
                 self.block_start_margin.union(block_start);
                 self.state = MarginCollapseState::AccumulatingMarginIn;
                 Au(0)
             }
-            (MarginCollapseState::AccumulatingMarginIn, CollapsibleMargins::None(block_start, _)) => {
+            (MarginCollapseState::AccumulatingMarginIn,
+             CollapsibleMargins::None(block_start, _)) => {
                 let previous_margin_value = self.margin_in.collapse();
                 self.margin_in = AdjoiningMargins::new();
                 previous_margin_value + block_start
diff --git a/tests/ref/basic.list b/tests/ref/basic.list
index 1d814a10e87..f7e535872ef 100644
--- a/tests/ref/basic.list
+++ b/tests/ref/basic.list
@@ -100,6 +100,7 @@ flaky_cpu == append_style_a.html append_style_b.html
 == float_intrinsic_width_a.html float_intrinsic_width_ref.html
 == float_right_intrinsic_width_a.html float_right_intrinsic_width_ref.html
 == float_table_a.html float_table_ref.html
+== float_under_top_margin_a.html float_under_top_margin_ref.html
 == floated_generated_content_a.html floated_generated_content_b.html
 == floated_list_item_a.html floated_list_item_ref.html
 == floated_table_with_margin_a.html floated_table_with_margin_ref.html
diff --git a/tests/ref/float_intrinsic_height.html b/tests/ref/float_intrinsic_height.html
index b6d9e7e302a..f349085d23c 100644
--- a/tests/ref/float_intrinsic_height.html
+++ b/tests/ref/float_intrinsic_height.html
@@ -19,6 +19,6 @@
 		<div class="otherprojects">
 			<div class="otherprojects-item">
 			</div>
-		</div>
+		</div>&nbsp;
 	</body>
 </html>
diff --git a/tests/ref/float_under_top_margin_a.html b/tests/ref/float_under_top_margin_a.html
new file mode 100644
index 00000000000..5cf1fdbdbd4
--- /dev/null
+++ b/tests/ref/float_under_top_margin_a.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<html>
+<head>
+<style>
+* {
+    margin: 0;
+    padding: 0;
+}
+body {
+    margin-top: 64px;
+}
+#foo, #bar {
+    height: 35px;
+    width: 35px;
+}
+#foo {
+    background: blue;
+    float: right;
+}
+#bar {
+    background: gold;
+}
+</style>
+</head>
+<body>
+<div id=foo></div><div id=bar></div>
+</body>
+</html>
+
diff --git a/tests/ref/float_under_top_margin_ref.html b/tests/ref/float_under_top_margin_ref.html
new file mode 100644
index 00000000000..904cbd5c66d
--- /dev/null
+++ b/tests/ref/float_under_top_margin_ref.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<html>
+<head>
+<style>
+* {
+    margin: 0;
+    padding: 0;
+}
+body {
+    padding-top: 64px;
+}
+#foo, #bar {
+    height: 35px;
+    width: 35px;
+}
+#foo {
+    background: blue;
+    float: right;
+}
+#bar {
+    background: gold;
+}
+</style>
+</head>
+<body>
+<div id=foo></div><div id=bar></div>
+</body>
+</html>
+
diff --git a/tests/ref/margins_inside_floats_a.html b/tests/ref/margins_inside_floats_a.html
index 139f12677df..9ad6aa576ed 100644
--- a/tests/ref/margins_inside_floats_a.html
+++ b/tests/ref/margins_inside_floats_a.html
@@ -4,7 +4,7 @@
 <div style="float: left;">
     <div style="margin-bottom: 64px;">Must be this tall</div>
     <div style="margin-top: 64px;">to write multi-threaded code.</div>
-</div>
+</div>&nbsp;
 </body>
 </html>