diff --git a/components/layout/block.rs b/components/layout/block.rs
index e573e422448..7c54df6e17c 100644
--- a/components/layout/block.rs
+++ b/components/layout/block.rs
@@ -327,8 +327,12 @@ impl CandidateBSizeIterator {
             (LengthOrPercentageOrAuto::Percentage(percent), Some(block_container_block_size)) => {
                 MaybeAuto::Specified(block_container_block_size.scale_by(percent))
             }
+            (LengthOrPercentageOrAuto::Calc(calc), Some(block_container_block_size)) => {
+                MaybeAuto::Specified(calc.length() + block_container_block_size.scale_by(calc.percentage()))
+            }
             (LengthOrPercentageOrAuto::Percentage(_), None) | (LengthOrPercentageOrAuto::Auto, _) => MaybeAuto::Auto,
             (LengthOrPercentageOrAuto::Length(length), _) => MaybeAuto::Specified(length),
+            (LengthOrPercentageOrAuto::Calc(calc), _) => MaybeAuto::Specified(calc.length()),
         };
         let max_block_size = match (fragment.style.max_block_size(), block_container_block_size) {
             (LengthOrPercentageOrNone::Percentage(percent), Some(block_container_block_size)) =>
@@ -1118,6 +1122,12 @@ impl BlockFlow {
         let content_block_size = self.fragment.style().content_block_size();
 
         match (content_block_size, containing_block_size) {
+            (LengthOrPercentageOrAuto::Calc(calc), Some(container_size)) => {
+                Some(container_size.scale_by(calc.percentage()) + calc.length())
+            }
+            (LengthOrPercentageOrAuto::Calc(calc), _) => {
+                Some(calc.length())
+            },
             (LengthOrPercentageOrAuto::Length(length), _) => Some(length),
             (LengthOrPercentageOrAuto::Percentage(percent), Some(container_size)) => {
                 Some(container_size.scale_by(percent))
diff --git a/components/layout/fragment.rs b/components/layout/fragment.rs
index 02af5569d84..8e3c92cad5e 100644
--- a/components/layout/fragment.rs
+++ b/components/layout/fragment.rs
@@ -463,6 +463,12 @@ impl ReplacedImageFragmentInfo {
                 MaybeAuto::Specified(container_size.scale_by(pc))
             }
             (LengthOrPercentageOrAuto::Percentage(_), _, None) => MaybeAuto::Auto,
+            (LengthOrPercentageOrAuto::Calc(calc), _, Some(container_size)) => {
+                MaybeAuto::Specified(calc.length() + container_size.scale_by(calc.percentage()))
+            }
+            (LengthOrPercentageOrAuto::Calc(calc), _, None) => {
+                MaybeAuto::Specified(calc.length())
+            }
             (LengthOrPercentageOrAuto::Auto, Some(dom_length), _) => MaybeAuto::Specified(dom_length),
             (LengthOrPercentageOrAuto::Auto, None, _) => MaybeAuto::Auto,
         }
@@ -616,6 +622,10 @@ impl IframeFragmentInfo {
         let computed_size = match (content_size, containing_size) {
             (LengthOrPercentageOrAuto::Length(length), _) => length,
             (LengthOrPercentageOrAuto::Percentage(pc), Some(container_size)) => container_size.scale_by(pc),
+            (LengthOrPercentageOrAuto::Calc(calc), Some(container_size)) => {
+                container_size.scale_by(calc.percentage()) + calc.length()
+            },
+            (LengthOrPercentageOrAuto::Calc(calc), None) => calc.length(),
             (LengthOrPercentageOrAuto::Percentage(_), None) => default_size,
             (LengthOrPercentageOrAuto::Auto, _) => default_size,
         };
@@ -1285,6 +1295,7 @@ impl Fragment {
                     }
                     (Some(dom_inline_size), _) => dom_inline_size,
                     (None, LengthOrPercentageOrAuto::Length(length)) => length,
+                    (None, LengthOrPercentageOrAuto::Calc(calc)) => calc.length(),
                 };
                 result.union_block(&IntrinsicISizes {
                     minimum_inline_size: image_inline_size,
diff --git a/components/layout/model.rs b/components/layout/model.rs
index ca50b7790d3..45efadf1246 100644
--- a/components/layout/model.rs
+++ b/components/layout/model.rs
@@ -379,6 +379,9 @@ impl MaybeAuto {
             LengthOrPercentageOrAuto::Percentage(percent) => {
                 MaybeAuto::Specified(containing_length.scale_by(percent))
             }
+            LengthOrPercentageOrAuto::Calc(calc) => {
+                MaybeAuto::Specified(calc.length() + containing_length.scale_by(calc.percentage()))
+            }
             LengthOrPercentageOrAuto::Length(length) => MaybeAuto::Specified(length)
         }
     }
diff --git a/components/layout/table.rs b/components/layout/table.rs
index f5ef95f1989..2e351b117fa 100644
--- a/components/layout/table.rs
+++ b/components/layout/table.rs
@@ -267,11 +267,13 @@ impl Flow for TableFlow {
                         self.column_intrinsic_inline_sizes.push(ColumnIntrinsicInlineSize {
                             minimum_length: match *specified_inline_size {
                                 LengthOrPercentageOrAuto::Auto |
+                                LengthOrPercentageOrAuto::Calc(_) |
                                 LengthOrPercentageOrAuto::Percentage(_) => Au(0),
                                 LengthOrPercentageOrAuto::Length(length) => length,
                             },
                             percentage: match *specified_inline_size {
                                 LengthOrPercentageOrAuto::Auto |
+                                LengthOrPercentageOrAuto::Calc(_) |
                                 LengthOrPercentageOrAuto::Length(_) => 0.0,
                                 LengthOrPercentageOrAuto::Percentage(percentage) => percentage,
                             },
diff --git a/components/layout/table_row.rs b/components/layout/table_row.rs
index e2b155bca78..eb02eade5b1 100644
--- a/components/layout/table_row.rs
+++ b/components/layout/table_row.rs
@@ -272,6 +272,7 @@ impl Flow for TableRowFlow {
                 let child_column_inline_size = ColumnIntrinsicInlineSize {
                     minimum_length: match child_specified_inline_size {
                         LengthOrPercentageOrAuto::Auto |
+                        LengthOrPercentageOrAuto::Calc(_) |
                         LengthOrPercentageOrAuto::Percentage(_) => {
                             child_base.intrinsic_inline_sizes.minimum_inline_size
                         }
@@ -279,6 +280,7 @@ impl Flow for TableRowFlow {
                     },
                     percentage: match child_specified_inline_size {
                         LengthOrPercentageOrAuto::Auto |
+                        LengthOrPercentageOrAuto::Calc(_) |
                         LengthOrPercentageOrAuto::Length(_) => 0.0,
                         LengthOrPercentageOrAuto::Percentage(percentage) => percentage,
                     },
@@ -286,6 +288,7 @@ impl Flow for TableRowFlow {
                     constrained: match child_specified_inline_size {
                         LengthOrPercentageOrAuto::Length(_) => true,
                         LengthOrPercentageOrAuto::Auto |
+                        LengthOrPercentageOrAuto::Calc(_) |
                         LengthOrPercentageOrAuto::Percentage(_) => false,
                     },
                 };
diff --git a/components/style/values.rs b/components/style/values.rs
index a3dcac5a81a..c44190fff9d 100644
--- a/components/style/values.rs
+++ b/components/style/values.rs
@@ -490,6 +490,7 @@ pub mod specified {
         Length(Length),
         Percentage(CSSFloat),  // [0 .. 100%] maps to [0.0 .. 1.0]
         Auto,
+        Calc(Calc),
     }
 
     impl ToCss for LengthOrPercentageOrAuto {
@@ -499,6 +500,7 @@ pub mod specified {
                 &LengthOrPercentageOrAuto::Percentage(percentage)
                 => write!(dest, "{}%", percentage * 100.),
                 &LengthOrPercentageOrAuto::Auto => dest.write_str("auto"),
+                &LengthOrPercentageOrAuto::Calc(calc) => calc.to_css(dest),
             }
         }
     }
@@ -516,6 +518,10 @@ pub mod specified {
                     Ok(LengthOrPercentageOrAuto::Length(Length::Absolute(Au(0)))),
                 Token::Ident(ref value) if value.eq_ignore_ascii_case("auto") =>
                     Ok(LengthOrPercentageOrAuto::Auto),
+                Token::Function(ref name) if name.eq_ignore_ascii_case("calc") => {
+                    let calc = try!(input.parse_nested_block(Calc::parse));
+                    Ok(LengthOrPercentageOrAuto::Calc(calc))
+                },
                 _ => Err(())
             }
         }
@@ -1122,6 +1128,7 @@ pub mod computed {
         Length(Au),
         Percentage(CSSFloat),
         Auto,
+        Calc(Calc),
     }
     impl fmt::Debug for LengthOrPercentageOrAuto {
         fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
@@ -1129,6 +1136,8 @@ pub mod computed {
                 &LengthOrPercentageOrAuto::Length(length) => write!(f, "{:?}", length),
                 &LengthOrPercentageOrAuto::Percentage(percentage) => write!(f, "{}%", percentage * 100.),
                 &LengthOrPercentageOrAuto::Auto => write!(f, "auto"),
+                // XXX HACK WRONG
+                &LengthOrPercentageOrAuto::Calc(calc) => write!(f, "{}%", 100.),
             }
         }
     }
@@ -1148,6 +1157,9 @@ pub mod computed {
                 specified::LengthOrPercentageOrAuto::Auto => {
                     LengthOrPercentageOrAuto::Auto
                 }
+                specified::LengthOrPercentageOrAuto::Calc(calc) => {
+                    LengthOrPercentageOrAuto::Calc(calc.to_computed_value(context))
+                }
             }
         }
     }
@@ -1159,6 +1171,7 @@ pub mod computed {
                 &LengthOrPercentageOrAuto::Percentage(percentage)
                 => write!(dest, "{}%", percentage * 100.),
                 &LengthOrPercentageOrAuto::Auto => dest.write_str("auto"),
+                &LengthOrPercentageOrAuto::Calc(calc) => calc.to_css(dest),
             }
         }
     }
diff --git a/components/style/viewport.rs b/components/style/viewport.rs
index 15dcac2eefe..bb38050f9b4 100644
--- a/components/style/viewport.rs
+++ b/components/style/viewport.rs
@@ -437,6 +437,8 @@ impl ViewportConstraints {
                         LengthOrPercentageOrAuto::Percentage(value) =>
                             Some(initial_viewport.$dimension.scale_by(value)),
                         LengthOrPercentageOrAuto::Auto => None,
+                        // XXX WRONG HACK
+                        LengthOrPercentageOrAuto::Calc(calc) => None,
                     }
                 } else {
                     None