diff --git a/src/components/main/layout/block.rs b/src/components/main/layout/block.rs
index e2d45002f65..72ae49e6a93 100644
--- a/src/components/main/layout/block.rs
+++ b/src/components/main/layout/block.rs
@@ -42,7 +42,8 @@ use std::fmt;
 use std::mem;
 use std::num::Zero;
 use style::computed_values::{LPA_Auto, LPA_Length, LPA_Percentage, LPN_Length, LPN_None};
-use style::computed_values::{LPN_Percentage, LP_Length, LP_Percentage, display, float, overflow};
+use style::computed_values::{LPN_Percentage, LP_Length, LP_Percentage};
+use style::computed_values::{display, direction, float, overflow};
 use sync::Arc;
 
 /// Information specific to floated blocks.
@@ -1717,6 +1718,7 @@ pub struct WidthConstraintInput {
     pub right: MaybeAuto,
     pub available_width: Au,
     pub static_x_offset: Au,
+    pub direction: direction::T,
 }
 
 impl WidthConstraintInput {
@@ -1726,7 +1728,8 @@ impl WidthConstraintInput {
                left: MaybeAuto,
                right: MaybeAuto,
                available_width: Au,
-               static_x_offset: Au)
+               static_x_offset: Au,
+               direction: direction::T)
            -> WidthConstraintInput {
         WidthConstraintInput {
             computed_width: computed_width,
@@ -1736,6 +1739,7 @@ impl WidthConstraintInput {
             right: right,
             available_width: available_width,
             static_x_offset: static_x_offset,
+            direction: direction,
         }
     }
 }
@@ -1814,7 +1818,8 @@ pub trait WidthAndMarginsComputer {
                                          left,
                                          right,
                                          available_width,
-                                         block.static_x_offset());
+                                         block.static_x_offset(),
+                                         style.get_inheritedbox().direction);
     }
 
     /// Set the used values for width and margins got from the relevant constraint equation.
@@ -1940,9 +1945,12 @@ pub trait WidthAndMarginsComputer {
             // If direction is ltr, ignore the specified right margin and
             // solve for it.
             // If it is rtl, ignore the specified left margin.
-            // FIXME(eatkinson): this assumes the direction is ltr
-            (Specified(margin_l), Specified(width), Specified(_margin_r)) =>
-                (margin_l, width, available_width - (margin_l + width )),
+            (Specified(margin_l), Specified(width), Specified(margin_r)) => {
+                match input.direction {
+                    direction::ltr => (margin_l, width, available_width - (margin_l + width)),
+                    direction::rtl => (available_width - (margin_r + width), width, margin_r),
+                }
+            },
 
             // If exactly one value is 'auto', solve for it
             (Auto, Specified(width), Specified(margin_r)) =>
@@ -2004,6 +2012,7 @@ impl WidthAndMarginsComputer for AbsoluteNonReplaced {
             right,
             available_width,
             static_x_offset,
+            direction,
         } = input;
 
         // TODO: Check for direction of parent flow (NOT Containing Block)
@@ -2152,6 +2161,7 @@ impl WidthAndMarginsComputer for AbsoluteReplaced {
             right,
             available_width,
             static_x_offset,
+            direction,
         } = input;
         // TODO: Check for direction of static-position Containing Block (aka
         // parent flow, _not_ the actual Containing Block) when right-to-left
diff --git a/src/components/style/properties.rs.mako b/src/components/style/properties.rs.mako
index 62292004817..fdd98a5ef08 100644
--- a/src/components/style/properties.rs.mako
+++ b/src/components/style/properties.rs.mako
@@ -321,8 +321,14 @@ pub mod longhands {
     ${single_keyword("float", "none left right")}
     ${single_keyword("clear", "none left right both")}
 
+    ${new_style_struct("InheritedBox", is_inherited=True)}
+
+    ${single_keyword("direction", "ltr rtl")}
+
     // CSS 2.1, Section 10 - Visual formatting model details
 
+    ${switch_to_style_struct("Box")}
+
     ${predefined_type("width", "LengthOrPercentageOrAuto",
                       "computed::LPA_Auto",
                       "parse_non_negative")}
@@ -364,7 +370,7 @@ pub mod longhands {
                       "computed::LPN_None",
                       "parse_non_negative")}
 
-    ${new_style_struct("InheritedBox", is_inherited=True)}
+    ${switch_to_style_struct("InheritedBox")}
 
     <%self:single_component_value name="line-height">
         #[deriving(Clone)]
diff --git a/src/test/ref/basic.list b/src/test/ref/basic.list
index 047eab56c16..58215700bb6 100644
--- a/src/test/ref/basic.list
+++ b/src/test/ref/basic.list
@@ -74,3 +74,4 @@
 == pseudo_element_a.html pseudo_element_b.html
 == linebreak_simple_a.html linebreak_simple_b.html
 == linebreak_inline_span_a.html linebreak_inline_span_b.html
+== overconstrained_block.html overconstrained_block_ref.html
diff --git a/src/test/ref/overconstrained_block.html b/src/test/ref/overconstrained_block.html
new file mode 100644
index 00000000000..564232f4cd7
--- /dev/null
+++ b/src/test/ref/overconstrained_block.html
@@ -0,0 +1,15 @@
+<html>
+  <head>
+    <title>Block with over-contrained margins+borders+padding+width = containing block width</title>
+    <style>
+      body { width: 300px; margin: 0 }
+      p { background: green; width: 200px; height: 100px; margin: 20px 70px }
+    </style>
+  </head>
+  <body>
+    <p style="margin: 20px 70px"></p>
+    <p style="margin: 20px 70px; direction: rtl"></p>
+    <p style="margin: 20px 120px"></p>
+    <p style="margin: 20px 120px; direction: rtl"></p>
+  </body>
+</html>
diff --git a/src/test/ref/overconstrained_block_ref.html b/src/test/ref/overconstrained_block_ref.html
new file mode 100644
index 00000000000..12016be7cfe
--- /dev/null
+++ b/src/test/ref/overconstrained_block_ref.html
@@ -0,0 +1,15 @@
+<html>
+  <head>
+    <title>Block with over-contrained margins+borders+padding+width = containing block width</title>
+    <style>
+      body { width: 300px; margin: 0; }
+      p { background: green; width: 200px; height: 100px; margin: 0; position: absolute }
+    </style>
+  </head>
+  <body>
+    <p style="top: 20px; left: 70px"></p>
+    <p style="top: 140px; left: 30px"></p>
+    <p style="top: 260px; left: 120px"></p>
+    <p style="top: 380px; left: -20px"></p>
+  </body>
+</html>