diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 2ca775ed345..ed652138d69 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -4,7 +4,6 @@
 
 //! Element nodes.
 
-use app_units::Au;
 use cssparser::Color;
 use devtools_traits::AttrInfo;
 use dom::activation::Activatable;
@@ -455,18 +454,13 @@ impl LayoutElementHelpers for LayoutJS<Element> {
                                 font_family)])))));
         }
 
-        let font_size = if let Some(this) = self.downcast::<HTMLFontElement>() {
-            this.get_size()
-        } else {
-            None
-        };
+        let font_size = self.downcast::<HTMLFontElement>().and_then(|this| this.get_size());
 
         if let Some(font_size) = font_size {
             hints.push(from_declaration(
                 PropertyDeclaration::FontSize(
                     DeclaredValue::Value(
-                        font_size::SpecifiedValue(
-                            LengthOrPercentage::Length(font_size))))))
+                        font_size::SpecifiedValue(font_size.into())))))
         }
 
         let cellspacing = if let Some(this) = self.downcast::<HTMLTableElement>() {
@@ -476,7 +470,7 @@ impl LayoutElementHelpers for LayoutJS<Element> {
         };
 
         if let Some(cellspacing) = cellspacing {
-            let width_value = specified::Length::Absolute(Au::from_px(cellspacing as i32));
+            let width_value = specified::Length::from_px(cellspacing as f32);
             hints.push(from_declaration(
                 PropertyDeclaration::BorderSpacing(DeclaredValue::Value(
                     border_spacing::SpecifiedValue {
@@ -509,13 +503,12 @@ impl LayoutElementHelpers for LayoutJS<Element> {
         };
 
         if let Some(size) = size {
-            let value = specified::Length::ServoCharacterWidth(specified::CharacterWidth(size));
+            let value = specified::NoCalcLength::ServoCharacterWidth(specified::CharacterWidth(size));
             hints.push(from_declaration(
                 PropertyDeclaration::Width(DeclaredValue::Value(
                     specified::LengthOrPercentageOrAuto::Length(value)))));
         }
 
-
         let width = if let Some(this) = self.downcast::<HTMLIFrameElement>() {
             this.get_width()
         } else if let Some(this) = self.downcast::<HTMLImageElement>() {
@@ -541,7 +534,7 @@ impl LayoutElementHelpers for LayoutJS<Element> {
             }
             LengthOrPercentageOrAuto::Length(length) => {
                 let width_value = specified::LengthOrPercentageOrAuto::Length(
-                    specified::Length::Absolute(length));
+                    specified::NoCalcLength::Absolute(length));
                 hints.push(from_declaration(
                     PropertyDeclaration::Width(DeclaredValue::Value(width_value))));
             }
@@ -566,7 +559,7 @@ impl LayoutElementHelpers for LayoutJS<Element> {
             }
             LengthOrPercentageOrAuto::Length(length) => {
                 let height_value = specified::LengthOrPercentageOrAuto::Length(
-                    specified::Length::Absolute(length));
+                    specified::NoCalcLength::Absolute(length));
                 hints.push(from_declaration(
                     PropertyDeclaration::Height(DeclaredValue::Value(height_value))));
             }
@@ -588,13 +581,12 @@ impl LayoutElementHelpers for LayoutJS<Element> {
             // scrollbar size into consideration (but we don't have a scrollbar yet!)
             //
             // https://html.spec.whatwg.org/multipage/#textarea-effective-width
-            let value = specified::Length::ServoCharacterWidth(specified::CharacterWidth(cols));
+            let value = specified::NoCalcLength::ServoCharacterWidth(specified::CharacterWidth(cols));
             hints.push(from_declaration(
                 PropertyDeclaration::Width(DeclaredValue::Value(
                     specified::LengthOrPercentageOrAuto::Length(value)))));
         }
 
-
         let rows = if let Some(this) = self.downcast::<HTMLTextAreaElement>() {
             match this.get_rows() {
                 0 => None,
@@ -608,7 +600,7 @@ impl LayoutElementHelpers for LayoutJS<Element> {
             // TODO(mttr) This should take scrollbar size into consideration.
             //
             // https://html.spec.whatwg.org/multipage/#textarea-effective-height
-            let value = specified::Length::FontRelative(specified::FontRelativeLength::Em(rows as CSSFloat));
+            let value = specified::NoCalcLength::FontRelative(specified::FontRelativeLength::Em(rows as CSSFloat));
             hints.push(from_declaration(
                 PropertyDeclaration::Height(DeclaredValue::Value(
                         specified::LengthOrPercentageOrAuto::Length(value)))));
@@ -622,8 +614,7 @@ impl LayoutElementHelpers for LayoutJS<Element> {
         };
 
         if let Some(border) = border {
-            let width_value = specified::BorderWidth::from_length(
-                specified::Length::Absolute(Au::from_px(border as i32)));
+            let width_value = specified::BorderWidth::from_length(specified::Length::from_px(border as f32));
             hints.push(from_declaration(
                 PropertyDeclaration::BorderTopWidth(DeclaredValue::Value(width_value.clone()))));
             hints.push(from_declaration(
diff --git a/components/style/gecko/media_queries.rs b/components/style/gecko/media_queries.rs
index 342baa002f1..dd9f5a19643 100644
--- a/components/style/gecko/media_queries.rs
+++ b/components/style/gecko/media_queries.rs
@@ -236,8 +236,7 @@ impl MediaExpressionValue {
             nsMediaFeature_ValueType::eLength => {
                 debug_assert!(css_value.mUnit == nsCSSUnit::eCSSUnit_Pixel);
                 let pixels = css_value.float_unchecked();
-                Some(MediaExpressionValue::Length(
-                        specified::Length::Absolute(Au::from_f32_px(pixels))))
+                Some(MediaExpressionValue::Length(specified::Length::from_px(pixels)))
             }
             nsMediaFeature_ValueType::eInteger => {
                 let i = css_value.integer_unchecked();
diff --git a/components/style/properties/longhand/box.mako.rs b/components/style/properties/longhand/box.mako.rs
index 754afaf164d..1558e597ead 100644
--- a/components/style/properties/longhand/box.mako.rs
+++ b/components/style/properties/longhand/box.mako.rs
@@ -1328,7 +1328,7 @@ ${helpers.single_keyword("animation-fill-mode",
                         result.push(SpecifiedOperation::Translate(TranslateKind::Translate,
                                                                   tx,
                                                                   ty,
-                                                                  specified::Length::Absolute(Au(0))));
+                                                                  specified::Length::zero()));
                         Ok(())
                     }))
                 },
@@ -1339,7 +1339,7 @@ ${helpers.single_keyword("animation-fill-mode",
                             TranslateKind::TranslateX,
                             tx,
                             specified::LengthOrPercentage::zero(),
-                            specified::Length::Absolute(Au(0))));
+                            specified::Length::zero()));
                         Ok(())
                     }))
                 },
@@ -1350,7 +1350,7 @@ ${helpers.single_keyword("animation-fill-mode",
                             TranslateKind::TranslateY,
                             specified::LengthOrPercentage::zero(),
                             ty,
-                            specified::Length::Absolute(Au(0))));
+                            specified::Length::zero()));
                         Ok(())
                     }))
                 },
@@ -1761,7 +1761,7 @@ ${helpers.single_keyword("transform-style",
     use std::fmt;
     use style_traits::ToCss;
     use values::HasViewportPercentage;
-    use values::specified::{Length, LengthOrPercentage, Percentage};
+    use values::specified::{NoCalcLength, LengthOrPercentage, Percentage};
 
     pub mod computed_value {
         use properties::animated_properties::Interpolate;
@@ -1799,7 +1799,7 @@ ${helpers.single_keyword("transform-style",
     pub struct SpecifiedValue {
         horizontal: LengthOrPercentage,
         vertical: LengthOrPercentage,
-        depth: Length,
+        depth: NoCalcLength,
     }
 
     impl ToCss for computed_value::T {
@@ -1836,7 +1836,7 @@ ${helpers.single_keyword("transform-style",
         Ok(SpecifiedValue {
             horizontal: result.horizontal.unwrap_or(LengthOrPercentage::Percentage(Percentage(0.5))),
             vertical: result.vertical.unwrap_or(LengthOrPercentage::Percentage(Percentage(0.5))),
-            depth: result.depth.unwrap_or(Length::Absolute(Au(0))),
+            depth: result.depth.unwrap_or(NoCalcLength::zero()),
         })
     }
 
diff --git a/components/style/properties/longhand/effects.mako.rs b/components/style/properties/longhand/effects.mako.rs
index 3da52aa3db2..9bc62bc5e7d 100644
--- a/components/style/properties/longhand/effects.mako.rs
+++ b/components/style/properties/longhand/effects.mako.rs
@@ -280,10 +280,10 @@ ${helpers.predefined_type("opacity",
                 left = try!(parse_argument(context, input));
             }
             Ok(SpecifiedValue(Some(SpecifiedClipRect {
-                top: top.unwrap_or(Length::Absolute(Au(0))),
+                top: top.unwrap_or(Length::zero()),
                 right: right,
                 bottom: bottom,
-                left: left.unwrap_or(Length::Absolute(Au(0))),
+                left: left.unwrap_or(Length::zero()),
             })))
         })
     }
@@ -613,7 +613,7 @@ ${helpers.predefined_type("opacity",
 pub struct OriginParseResult {
     pub horizontal: Option<specified::LengthOrPercentage>,
     pub vertical: Option<specified::LengthOrPercentage>,
-    pub depth: Option<specified::Length>
+    pub depth: Option<specified::NoCalcLength>
 }
 
 pub fn parse_origin(context: &ParserContext, input: &mut Parser) -> Result<OriginParseResult,()> {
diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index d143398b2d8..b25d40fefea 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -329,7 +329,7 @@ ${helpers.single_keyword("font-variant-caps",
     use std::fmt;
     use style_traits::ToCss;
     use values::{FONT_MEDIUM_PX, HasViewportPercentage};
-    use values::specified::{LengthOrPercentage, Length, Percentage};
+    use values::specified::{LengthOrPercentage, Length, NoCalcLength, Percentage};
 
     impl ToCss for SpecifiedValue {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
@@ -363,10 +363,10 @@ ${helpers.single_keyword("font-variant-caps",
         #[inline]
         fn to_computed_value(&self, context: &Context) -> computed_value::T {
             match self.0 {
-                LengthOrPercentage::Length(Length::FontRelative(value)) => {
+                LengthOrPercentage::Length(NoCalcLength::FontRelative(value)) => {
                     value.to_computed_value(context, /* use inherited */ true)
                 }
-                LengthOrPercentage::Length(Length::ServoCharacterWidth(value)) => {
+                LengthOrPercentage::Length(NoCalcLength::ServoCharacterWidth(value)) => {
                     value.to_computed_value(context.inherited_style().get_font().clone_font_size())
                 }
                 LengthOrPercentage::Length(ref l) => {
@@ -397,11 +397,10 @@ ${helpers.single_keyword("font-variant-caps",
         input.try(specified::LengthOrPercentage::parse_non_negative)
         .or_else(|()| {
             let ident = try!(input.expect_ident());
-            specified::Length::from_str(&ident as &str)
-                .ok_or(())
-                .map(specified::LengthOrPercentage::Length)
-        })
-        .map(SpecifiedValue)
+            NoCalcLength::from_str(&ident as &str)
+                         .ok_or(())
+                         .map(specified::LengthOrPercentage::Length)
+        }).map(SpecifiedValue)
     }
 </%helpers:longhand>
 
diff --git a/components/style/properties/longhand/inherited_text.mako.rs b/components/style/properties/longhand/inherited_text.mako.rs
index db83c7308df..9c63fd45686 100644
--- a/components/style/properties/longhand/inherited_text.mako.rs
+++ b/components/style/properties/longhand/inherited_text.mako.rs
@@ -113,13 +113,14 @@
                         specified::LengthOrPercentage::Length(ref value) =>
                             computed_value::T::Length(value.to_computed_value(context)),
                         specified::LengthOrPercentage::Percentage(specified::Percentage(value)) => {
-                            let fr = specified::Length::FontRelative(specified::FontRelativeLength::Em(value));
+                            let fr = specified::Length::NoCalc(specified::NoCalcLength::FontRelative(
+                                specified::FontRelativeLength::Em(value)));
                             computed_value::T::Length(fr.to_computed_value(context))
                         },
                         specified::LengthOrPercentage::Calc(ref calc) => {
                             let calc = calc.to_computed_value(context);
                             let fr = specified::FontRelativeLength::Em(calc.percentage());
-                            let fr = specified::Length::FontRelative(fr);
+                            let fr = specified::Length::NoCalc(specified::NoCalcLength::FontRelative(fr));
                             computed_value::T::Length(calc.length() + fr.to_computed_value(context))
                         }
                     }
@@ -681,9 +682,7 @@ ${helpers.single_keyword("text-align-last",
 
     fn parse_one_text_shadow(context: &ParserContext, input: &mut Parser) -> Result<SpecifiedTextShadow,()> {
         use app_units::Au;
-        let mut lengths = [specified::Length::Absolute(Au(0)),
-                           specified::Length::Absolute(Au(0)),
-                           specified::Length::Absolute(Au(0))];
+        let mut lengths = [specified::Length::zero(), specified::Length::zero(), specified::Length::zero()];
         let mut lengths_parsed = false;
         let mut color = None;
 
diff --git a/components/style/properties/shorthand/inherited_text.mako.rs b/components/style/properties/shorthand/inherited_text.mako.rs
index a75e6d8925d..f97c129590d 100644
--- a/components/style/properties/shorthand/inherited_text.mako.rs
+++ b/components/style/properties/shorthand/inherited_text.mako.rs
@@ -74,7 +74,6 @@
 
     pub fn parse_value(context: &ParserContext, input: &mut Parser) -> Result<Longhands, ()> {
         use values::specified::{BorderWidth, Length};
-        use app_units::Au;
 
         let mut color = None;
         let mut width = None;
@@ -99,7 +98,7 @@
             Ok(Longhands {
                 _webkit_text_stroke_color: color.or(Some(CSSColor { parsed: CSSParserColor::CurrentColor,
                     authored: None })),
-                _webkit_text_stroke_width: width.or(Some(BorderWidth::from_length(Length::Absolute(Au::from_px(0))))),
+                _webkit_text_stroke_width: width.or(Some(BorderWidth::from_length(Length::zero()))),
             })
         } else {
             Err(())
diff --git a/components/style/properties/shorthand/position.mako.rs b/components/style/properties/shorthand/position.mako.rs
index 807bcadb5df..b5673925996 100644
--- a/components/style/properties/shorthand/position.mako.rs
+++ b/components/style/properties/shorthand/position.mako.rs
@@ -57,8 +57,7 @@
 <%helpers:shorthand name="flex" sub_properties="flex-grow flex-shrink flex-basis" extra_prefixes="webkit"
                     spec="https://drafts.csswg.org/css-flexbox/#flex-property">
     use parser::Parse;
-    use app_units::Au;
-    use values::specified::{Number, Length, LengthOrPercentageOrAutoOrContent};
+    use values::specified::{Number, NoCalcLength, LengthOrPercentageOrAutoOrContent};
 
     pub fn parse_flexibility(input: &mut Parser)
                              -> Result<(Number, Option<Number>),()> {
@@ -102,7 +101,7 @@
         Ok(Longhands {
             flex_grow: grow.or(Some(Number(1.0))),
             flex_shrink: shrink.or(Some(Number(1.0))),
-            flex_basis: basis.or(Some(LengthOrPercentageOrAutoOrContent::Length(Length::Absolute(Au(0)))))
+            flex_basis: basis.or(Some(LengthOrPercentageOrAutoOrContent::Length(NoCalcLength::zero())))
         })
     }
 
diff --git a/components/style/values/computed/length.rs b/components/style/values/computed/length.rs
index b2a3a983e0d..74c9fd6287f 100644
--- a/components/style/values/computed/length.rs
+++ b/components/style/values/computed/length.rs
@@ -17,6 +17,45 @@ pub use super::image::{EndingShape as GradientShape, Gradient, GradientKind, Ima
 pub use super::image::{LengthOrKeyword, LengthOrPercentageOrKeyword};
 pub use values::specified::{Angle, BorderStyle, Time, UrlOrNone};
 
+impl ToComputedValue for specified::NoCalcLength {
+    type ComputedValue = Au;
+
+    #[inline]
+    fn to_computed_value(&self, context: &Context) -> Au {
+        match *self {
+            specified::NoCalcLength::Absolute(length) => length,
+            specified::NoCalcLength::FontRelative(length) =>
+                length.to_computed_value(context, /* use inherited */ false),
+            specified::NoCalcLength::ViewportPercentage(length) =>
+                length.to_computed_value(context.viewport_size()),
+            specified::NoCalcLength::ServoCharacterWidth(length) =>
+                length.to_computed_value(context.style().get_font().clone_font_size())
+        }
+    }
+
+    #[inline]
+    fn from_computed_value(computed: &Au) -> Self {
+        specified::NoCalcLength::Absolute(*computed)
+    }
+}
+
+impl ToComputedValue for specified::Length {
+    type ComputedValue = Au;
+
+    #[inline]
+    fn to_computed_value(&self, context: &Context) -> Au {
+        match *self {
+            specified::Length::NoCalc(l) => l.to_computed_value(context),
+            specified::Length::Calc(ref calc, range) => range.clamp(calc.to_computed_value(context).length()),
+        }
+    }
+
+    #[inline]
+    fn from_computed_value(computed: &Au) -> Self {
+        specified::Length::NoCalc(specified::NoCalcLength::from_computed_value(computed))
+    }
+}
+
 #[derive(Clone, PartialEq, Copy, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index 15aa9ceaf25..a653d18918c 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -121,30 +121,6 @@ impl ToComputedValue for specified::CSSColor {
 
 impl ComputedValueAsSpecified for specified::BorderStyle {}
 
-impl ToComputedValue for specified::Length {
-    type ComputedValue = Au;
-
-    #[inline]
-    fn to_computed_value(&self, context: &Context) -> Au {
-        match *self {
-            specified::Length::Absolute(length) => length,
-            specified::Length::Calc(ref calc, range) => range.clamp(calc.to_computed_value(context).length()),
-            specified::Length::FontRelative(length) =>
-                length.to_computed_value(context, /* use inherited */ false),
-            specified::Length::ViewportPercentage(length) =>
-                length.to_computed_value(context.viewport_size()),
-            specified::Length::ServoCharacterWidth(length) =>
-                length.to_computed_value(context.style().get_font().clone_font_size())
-        }
-    }
-
-    #[inline]
-    fn from_computed_value(computed: &Au) -> Self {
-        specified::Length::Absolute(*computed)
-    }
-}
-
-
 #[derive(Debug, PartialEq, Clone, Copy)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
diff --git a/components/style/values/specified/basic_shape.rs b/components/style/values/specified/basic_shape.rs
index d89f926dee4..7f77337218c 100644
--- a/components/style/values/specified/basic_shape.rs
+++ b/components/style/values/specified/basic_shape.rs
@@ -299,7 +299,6 @@ impl ToComputedValue for InsetRect {
 /// the keywords are folded into the percentages
 fn serialize_basicshape_position<W>(position: &Position, dest: &mut W)
     -> fmt::Result where W: fmt::Write {
-        use values::specified::Length;
         use values::specified::position::Keyword;
 
         // keyword-percentage pairs can be folded into a single percentage
@@ -327,7 +326,7 @@ fn serialize_basicshape_position<W>(position: &Position, dest: &mut W)
         // 0 length should be replaced with 0%
         fn replace_with_percent(input: LengthOrPercentage) -> LengthOrPercentage {
             match input {
-                LengthOrPercentage::Length(Length::Absolute(au)) if au.0 == 0 => {
+                LengthOrPercentage::Length(ref l) if l.is_zero() => {
                     LengthOrPercentage::Percentage(Percentage(0.0))
                 }
                 _ => {
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 5f53376bb09..601056d6447 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -24,6 +24,14 @@ pub use super::image::{AngleOrCorner, ColorStop, EndingShape as GradientEndingSh
 pub use super::image::{GradientKind, HorizontalDirection, Image, LengthOrKeyword, LengthOrPercentageOrKeyword};
 pub use super::image::{SizeKeyword, VerticalDirection};
 
+const AU_PER_PX: CSSFloat = 60.;
+const AU_PER_IN: CSSFloat = AU_PER_PX * 96.;
+const AU_PER_CM: CSSFloat = AU_PER_IN / 2.54;
+const AU_PER_MM: CSSFloat = AU_PER_IN / 25.4;
+const AU_PER_Q: CSSFloat = AU_PER_MM / 4.;
+const AU_PER_PT: CSSFloat = AU_PER_IN / 72.;
+const AU_PER_PC: CSSFloat = AU_PER_PT * 12.;
+
 #[derive(Clone, PartialEq, Copy, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 /// A font relative length.
@@ -205,12 +213,12 @@ impl CharacterWidth {
     }
 }
 
-/// A length.
+/// A `<length>` without taking `calc` expressions into account
 ///
 /// https://drafts.csswg.org/css-values/#lengths
-#[derive(Clone, PartialEq, Debug)]
+#[derive(Clone, PartialEq, Copy, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-pub enum Length {
+pub enum NoCalcLength {
     /// An absolute length: https://drafts.csswg.org/css-values/#absolute-length
     Absolute(Au),  // application units
 
@@ -229,7 +237,114 @@ pub enum Length {
     /// This cannot be specified by the user directly and is only generated by
     /// `Stylist::synthesize_rules_for_legacy_attributes()`.
     ServoCharacterWidth(CharacterWidth),
+}
 
+impl HasViewportPercentage for NoCalcLength {
+    fn has_viewport_percentage(&self) -> bool {
+        match *self {
+            NoCalcLength::ViewportPercentage(_) => true,
+            _ => false,
+        }
+    }
+}
+
+impl ToCss for NoCalcLength {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+        match *self {
+            NoCalcLength::Absolute(length) => write!(dest, "{}px", length.to_f32_px()),
+            NoCalcLength::FontRelative(length) => length.to_css(dest),
+            NoCalcLength::ViewportPercentage(length) => length.to_css(dest),
+            /* This should only be reached from style dumping code */
+            NoCalcLength::ServoCharacterWidth(CharacterWidth(i)) => write!(dest, "CharWidth({})", i),
+        }
+    }
+}
+
+impl Mul<CSSFloat> for NoCalcLength {
+    type Output = NoCalcLength;
+
+    #[inline]
+    fn mul(self, scalar: CSSFloat) -> NoCalcLength {
+        match self {
+            NoCalcLength::Absolute(Au(v)) => NoCalcLength::Absolute(Au(((v as f32) * scalar) as i32)),
+            NoCalcLength::FontRelative(v) => NoCalcLength::FontRelative(v * scalar),
+            NoCalcLength::ViewportPercentage(v) => NoCalcLength::ViewportPercentage(v * scalar),
+            NoCalcLength::ServoCharacterWidth(_) => panic!("Can't multiply ServoCharacterWidth!"),
+        }
+    }
+}
+
+impl NoCalcLength {
+    /// https://drafts.csswg.org/css-fonts-3/#font-size-prop
+    pub fn from_str(s: &str) -> Option<NoCalcLength> {
+        Some(match_ignore_ascii_case! { s,
+            "xx-small" => NoCalcLength::Absolute(Au::from_px(FONT_MEDIUM_PX) * 3 / 5),
+            "x-small" => NoCalcLength::Absolute(Au::from_px(FONT_MEDIUM_PX) * 3 / 4),
+            "small" => NoCalcLength::Absolute(Au::from_px(FONT_MEDIUM_PX) * 8 / 9),
+            "medium" => NoCalcLength::Absolute(Au::from_px(FONT_MEDIUM_PX)),
+            "large" => NoCalcLength::Absolute(Au::from_px(FONT_MEDIUM_PX) * 6 / 5),
+            "x-large" => NoCalcLength::Absolute(Au::from_px(FONT_MEDIUM_PX) * 3 / 2),
+            "xx-large" => NoCalcLength::Absolute(Au::from_px(FONT_MEDIUM_PX) * 2),
+
+            // https://github.com/servo/servo/issues/3423#issuecomment-56321664
+            "smaller" => NoCalcLength::FontRelative(FontRelativeLength::Em(0.85)),
+            "larger" => NoCalcLength::FontRelative(FontRelativeLength::Em(1.2)),
+            _ => return None
+        })
+    }
+
+    /// Parse a given absolute or relative dimension.
+    pub fn parse_dimension(value: CSSFloat, unit: &str) -> Result<NoCalcLength, ()> {
+        match_ignore_ascii_case! { unit,
+            "px" => Ok(NoCalcLength::Absolute(Au((value * AU_PER_PX) as i32))),
+            "in" => Ok(NoCalcLength::Absolute(Au((value * AU_PER_IN) as i32))),
+            "cm" => Ok(NoCalcLength::Absolute(Au((value * AU_PER_CM) as i32))),
+            "mm" => Ok(NoCalcLength::Absolute(Au((value * AU_PER_MM) as i32))),
+            "q" => Ok(NoCalcLength::Absolute(Au((value * AU_PER_Q) as i32))),
+            "pt" => Ok(NoCalcLength::Absolute(Au((value * AU_PER_PT) as i32))),
+            "pc" => Ok(NoCalcLength::Absolute(Au((value * AU_PER_PC) as i32))),
+            // font-relative
+            "em" => Ok(NoCalcLength::FontRelative(FontRelativeLength::Em(value))),
+            "ex" => Ok(NoCalcLength::FontRelative(FontRelativeLength::Ex(value))),
+            "ch" => Ok(NoCalcLength::FontRelative(FontRelativeLength::Ch(value))),
+            "rem" => Ok(NoCalcLength::FontRelative(FontRelativeLength::Rem(value))),
+            // viewport percentages
+            "vw" => Ok(NoCalcLength::ViewportPercentage(ViewportPercentageLength::Vw(value))),
+            "vh" => Ok(NoCalcLength::ViewportPercentage(ViewportPercentageLength::Vh(value))),
+            "vmin" => Ok(NoCalcLength::ViewportPercentage(ViewportPercentageLength::Vmin(value))),
+            "vmax" => Ok(NoCalcLength::ViewportPercentage(ViewportPercentageLength::Vmax(value))),
+            _ => Err(())
+        }
+    }
+
+    #[inline]
+    /// Returns a `zero` length.
+    pub fn zero() -> NoCalcLength {
+        NoCalcLength::Absolute(Au(0))
+    }
+
+    #[inline]
+    /// Checks whether the length value is zero.
+    pub fn is_zero(&self) -> bool {
+        *self == NoCalcLength::Absolute(Au(0))
+    }
+
+    /// Get an absolute length from a px value.
+    #[inline]
+    pub fn from_px(px_value: CSSFloat) -> NoCalcLength {
+        NoCalcLength::Absolute(Au((px_value * AU_PER_PX) as i32))
+    }
+}
+
+/// An extension to `NoCalcLength` to parse `calc` expressions.
+/// This is commonly used for the `<length>` values.
+///
+/// https://drafts.csswg.org/css-values/#lengths
+#[derive(Clone, PartialEq, Debug)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+pub enum Length {
+    /// The internal length type that cannot parse `calc`
+    NoCalc(NoCalcLength),
     /// A calc expression.
     ///
     /// https://drafts.csswg.org/css-values/#calc-notation
@@ -242,9 +357,8 @@ pub enum Length {
 impl HasViewportPercentage for Length {
     fn has_viewport_percentage(&self) -> bool {
         match *self {
-            Length::ViewportPercentage(_) => true,
+            Length::NoCalc(ref inner) => inner.has_viewport_percentage(),
             Length::Calc(ref calc, _) => calc.has_viewport_percentage(),
-            _ => false
         }
     }
 }
@@ -252,12 +366,8 @@ impl HasViewportPercentage for Length {
 impl ToCss for Length {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
         match *self {
-            Length::Absolute(length) => write!(dest, "{}px", length.to_f32_px()),
-            Length::FontRelative(length) => length.to_css(dest),
-            Length::ViewportPercentage(length) => length.to_css(dest),
+            Length::NoCalc(ref inner) => inner.to_css(dest),
             Length::Calc(ref calc, _) => calc.to_css(dest),
-            /* This should only be reached from style dumping code */
-            Length::ServoCharacterWidth(CharacterWidth(i)) => write!(dest, "CharWidth({})", i),
         }
     }
 }
@@ -268,11 +378,8 @@ impl Mul<CSSFloat> for Length {
     #[inline]
     fn mul(self, scalar: CSSFloat) -> Length {
         match self {
-            Length::Absolute(Au(v)) => Length::Absolute(Au(((v as f32) * scalar) as i32)),
-            Length::FontRelative(v) => Length::FontRelative(v * scalar),
-            Length::ViewportPercentage(v) => Length::ViewportPercentage(v * scalar),
+            Length::NoCalc(inner) => Length::NoCalc(inner * scalar),
             Length::Calc(..) => panic!("Can't multiply Calc!"),
-            Length::ServoCharacterWidth(_) => panic!("Can't multiply ServoCharacterWidth!"),
         }
     }
 }
@@ -305,31 +412,21 @@ impl Mul<CSSFloat> for ViewportPercentageLength {
     }
 }
 
-const AU_PER_PX: CSSFloat = 60.;
-const AU_PER_IN: CSSFloat = AU_PER_PX * 96.;
-const AU_PER_CM: CSSFloat = AU_PER_IN / 2.54;
-const AU_PER_MM: CSSFloat = AU_PER_IN / 25.4;
-const AU_PER_Q: CSSFloat = AU_PER_MM / 4.;
-const AU_PER_PT: CSSFloat = AU_PER_IN / 72.;
-const AU_PER_PC: CSSFloat = AU_PER_PT * 12.;
-
 impl Length {
+    #[inline]
+    /// Returns a `zero` length.
+    pub fn zero() -> Length {
+        Length::NoCalc(NoCalcLength::zero())
+    }
+
     /// https://drafts.csswg.org/css-fonts-3/#font-size-prop
     pub fn from_str(s: &str) -> Option<Length> {
-        Some(match_ignore_ascii_case! { s,
-            "xx-small" => Length::Absolute(Au::from_px(FONT_MEDIUM_PX) * 3 / 5),
-            "x-small" => Length::Absolute(Au::from_px(FONT_MEDIUM_PX) * 3 / 4),
-            "small" => Length::Absolute(Au::from_px(FONT_MEDIUM_PX) * 8 / 9),
-            "medium" => Length::Absolute(Au::from_px(FONT_MEDIUM_PX)),
-            "large" => Length::Absolute(Au::from_px(FONT_MEDIUM_PX) * 6 / 5),
-            "x-large" => Length::Absolute(Au::from_px(FONT_MEDIUM_PX) * 3 / 2),
-            "xx-large" => Length::Absolute(Au::from_px(FONT_MEDIUM_PX) * 2),
+        NoCalcLength::from_str(s).map(Length::NoCalc)
+    }
 
-            // https://github.com/servo/servo/issues/3423#issuecomment-56321664
-            "smaller" => Length::FontRelative(FontRelativeLength::Em(0.85)),
-            "larger" => Length::FontRelative(FontRelativeLength::Em(1.2)),
-            _ => return None
-        })
+    /// Parse a given absolute or relative dimension.
+    pub fn parse_dimension(value: CSSFloat, unit: &str) -> Result<Length, ()> {
+        NoCalcLength::parse_dimension(value, unit).map(Length::NoCalc)
     }
 
     #[inline]
@@ -337,8 +434,7 @@ impl Length {
         match try!(input.next()) {
             Token::Dimension(ref value, ref unit) if context.is_ok(value.value) =>
                 Length::parse_dimension(value.value, unit),
-            Token::Number(ref value) if value.value == 0. =>
-                Ok(Length::Absolute(Au(0))),
+            Token::Number(ref value) if value.value == 0. => Ok(Length::zero()),
             Token::Function(ref name) if name.eq_ignore_ascii_case("calc") =>
                 input.parse_nested_block(|input| {
                     CalcLengthOrPercentage::parse_length(input, context)
@@ -352,34 +448,10 @@ impl Length {
         Length::parse_internal(input, AllowedNumericType::NonNegative)
     }
 
-    /// Parse a given absolute or relative dimension.
-    pub fn parse_dimension(value: CSSFloat, unit: &str) -> Result<Length, ()> {
-        match_ignore_ascii_case! { unit,
-            "px" => Ok(Length::from_px(value)),
-            "in" => Ok(Length::Absolute(Au((value * AU_PER_IN) as i32))),
-            "cm" => Ok(Length::Absolute(Au((value * AU_PER_CM) as i32))),
-            "mm" => Ok(Length::Absolute(Au((value * AU_PER_MM) as i32))),
-            "q" => Ok(Length::Absolute(Au((value * AU_PER_Q) as i32))),
-            "pt" => Ok(Length::Absolute(Au((value * AU_PER_PT) as i32))),
-            "pc" => Ok(Length::Absolute(Au((value * AU_PER_PC) as i32))),
-            // font-relative
-            "em" => Ok(Length::FontRelative(FontRelativeLength::Em(value))),
-            "ex" => Ok(Length::FontRelative(FontRelativeLength::Ex(value))),
-            "ch" => Ok(Length::FontRelative(FontRelativeLength::Ch(value))),
-            "rem" => Ok(Length::FontRelative(FontRelativeLength::Rem(value))),
-            // viewport percentages
-            "vw" => Ok(Length::ViewportPercentage(ViewportPercentageLength::Vw(value))),
-            "vh" => Ok(Length::ViewportPercentage(ViewportPercentageLength::Vh(value))),
-            "vmin" => Ok(Length::ViewportPercentage(ViewportPercentageLength::Vmin(value))),
-            "vmax" => Ok(Length::ViewportPercentage(ViewportPercentageLength::Vmax(value))),
-            _ => Err(())
-        }
-    }
-
-    /// Get an absolute length from a px values.
+    /// Get an absolute length from a px value.
     #[inline]
     pub fn from_px(px_value: CSSFloat) -> Length {
-        Length::Absolute(Au((px_value * AU_PER_PX) as i32))
+        Length::NoCalc(NoCalcLength::from_px(px_value))
     }
 
     /// Extract inner length without a clone, replacing it with a 0 Au
@@ -387,8 +459,7 @@ impl Length {
     /// Use when you need to move out of a length array without cloning
     #[inline]
     pub fn take(&mut self) -> Self {
-        let new = Length::Absolute(Au(0));
-        mem::replace(self, new)
+        mem::replace(self, Length::zero())
     }
 }
 
@@ -424,7 +495,7 @@ pub struct CalcProductNode {
 #[derive(Clone, Debug)]
 #[allow(missing_docs)]
 pub enum CalcValueNode {
-    Length(Length),
+    Length(NoCalcLength),
     Angle(Angle),
     Time(Time),
     Percentage(CSSFloat),
@@ -517,7 +588,7 @@ impl CalcLengthOrPercentage {
             (Token::Number(ref value), _) => Ok(CalcValueNode::Number(value.value)),
             (Token::Dimension(ref value, ref unit), CalcUnit::Length) |
             (Token::Dimension(ref value, ref unit), CalcUnit::LengthOrPercentage) => {
-                Length::parse_dimension(value.value, unit).map(CalcValueNode::Length)
+                NoCalcLength::parse_dimension(value.value, unit).map(CalcValueNode::Length)
             }
             (Token::Dimension(ref value, ref unit), CalcUnit::Angle) => {
                 Angle::parse_dimension(value.value, unit).map(CalcValueNode::Angle)
@@ -648,9 +719,9 @@ impl CalcLengthOrPercentage {
             match value {
                 SimplifiedValueNode::Percentage(p) =>
                     percentage = Some(percentage.unwrap_or(0.) + p),
-                SimplifiedValueNode::Length(Length::Absolute(Au(au))) =>
+                SimplifiedValueNode::Length(NoCalcLength::Absolute(Au(au))) =>
                     absolute = Some(absolute.unwrap_or(0) + au),
-                SimplifiedValueNode::Length(Length::ViewportPercentage(v)) =>
+                SimplifiedValueNode::Length(NoCalcLength::ViewportPercentage(v)) =>
                     match v {
                         ViewportPercentageLength::Vw(val) =>
                             vw = Some(vw.unwrap_or(0.) + val),
@@ -661,7 +732,7 @@ impl CalcLengthOrPercentage {
                         ViewportPercentageLength::Vmax(val) =>
                             vmax = Some(vmax.unwrap_or(0.) + val),
                     },
-                SimplifiedValueNode::Length(Length::FontRelative(f)) =>
+                SimplifiedValueNode::Length(NoCalcLength::FontRelative(f)) =>
                     match f {
                         FontRelativeLength::Em(val) =>
                             em = Some(em.unwrap_or(0.) + val),
@@ -833,11 +904,20 @@ impl Parse for Percentage {
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
 pub enum LengthOrPercentage {
-    Length(Length),
+    Length(NoCalcLength),
     Percentage(Percentage),
     Calc(Box<CalcLengthOrPercentage>),
 }
 
+impl From<Length> for LengthOrPercentage {
+    fn from(len: Length) -> LengthOrPercentage {
+        match len {
+            Length::NoCalc(l) => LengthOrPercentage::Length(l),
+            Length::Calc(l, _) => LengthOrPercentage::Calc(l),
+        }
+    }
+}
+
 impl HasViewportPercentage for LengthOrPercentage {
     fn has_viewport_percentage(&self) -> bool {
         match *self {
@@ -860,7 +940,7 @@ impl ToCss for LengthOrPercentage {
 impl LengthOrPercentage {
     /// Returns a `zero` length.
     pub fn zero() -> LengthOrPercentage {
-        LengthOrPercentage::Length(Length::Absolute(Au(0)))
+        LengthOrPercentage::Length(NoCalcLength::zero())
     }
 
     fn parse_internal(input: &mut Parser, context: AllowedNumericType)
@@ -868,11 +948,11 @@ impl LengthOrPercentage {
     {
         match try!(input.next()) {
             Token::Dimension(ref value, ref unit) if context.is_ok(value.value) =>
-                Length::parse_dimension(value.value, unit).map(LengthOrPercentage::Length),
+                NoCalcLength::parse_dimension(value.value, unit).map(LengthOrPercentage::Length),
             Token::Percentage(ref value) if context.is_ok(value.unit_value) =>
                 Ok(LengthOrPercentage::Percentage(Percentage(value.unit_value))),
             Token::Number(ref value) if value.value == 0. =>
-                Ok(LengthOrPercentage::Length(Length::Absolute(Au(0)))),
+                Ok(LengthOrPercentage::zero()),
             Token::Function(ref name) if name.eq_ignore_ascii_case("calc") => {
                 let calc = try!(input.parse_nested_block(CalcLengthOrPercentage::parse_length_or_percentage));
                 Ok(LengthOrPercentage::Calc(Box::new(calc)))
@@ -892,8 +972,7 @@ impl LengthOrPercentage {
     /// Use when you need to move out of a length array without cloning
     #[inline]
     pub fn take(&mut self) -> Self {
-        let new = LengthOrPercentage::Length(Length::Absolute(Au(0)));
-        mem::replace(self, new)
+        mem::replace(self, LengthOrPercentage::zero())
     }
 }
 
@@ -910,7 +989,7 @@ impl Parse for LengthOrPercentage {
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
 pub enum LengthOrPercentageOrAuto {
-    Length(Length),
+    Length(NoCalcLength),
     Percentage(Percentage),
     Auto,
     Calc(Box<CalcLengthOrPercentage>),
@@ -943,11 +1022,11 @@ impl LengthOrPercentageOrAuto {
     {
         match try!(input.next()) {
             Token::Dimension(ref value, ref unit) if context.is_ok(value.value) =>
-                Length::parse_dimension(value.value, unit).map(LengthOrPercentageOrAuto::Length),
+                NoCalcLength::parse_dimension(value.value, unit).map(LengthOrPercentageOrAuto::Length),
             Token::Percentage(ref value) if context.is_ok(value.unit_value) =>
                 Ok(LengthOrPercentageOrAuto::Percentage(Percentage(value.unit_value))),
             Token::Number(ref value) if value.value == 0. =>
-                Ok(LengthOrPercentageOrAuto::Length(Length::Absolute(Au(0)))),
+                Ok(LengthOrPercentageOrAuto::Length(NoCalcLength::zero())),
             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") => {
@@ -978,7 +1057,7 @@ impl Parse for LengthOrPercentageOrAuto {
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
 pub enum LengthOrPercentageOrNone {
-    Length(Length),
+    Length(NoCalcLength),
     Percentage(Percentage),
     Calc(Box<CalcLengthOrPercentage>),
     None,
@@ -1010,11 +1089,11 @@ impl LengthOrPercentageOrNone {
     {
         match try!(input.next()) {
             Token::Dimension(ref value, ref unit) if context.is_ok(value.value) =>
-                Length::parse_dimension(value.value, unit).map(LengthOrPercentageOrNone::Length),
+                NoCalcLength::parse_dimension(value.value, unit).map(LengthOrPercentageOrNone::Length),
             Token::Percentage(ref value) if context.is_ok(value.unit_value) =>
                 Ok(LengthOrPercentageOrNone::Percentage(Percentage(value.unit_value))),
             Token::Number(ref value) if value.value == 0. =>
-                Ok(LengthOrPercentageOrNone::Length(Length::Absolute(Au(0)))),
+                Ok(LengthOrPercentageOrNone::Length(NoCalcLength::zero())),
             Token::Function(ref name) if name.eq_ignore_ascii_case("calc") => {
                 let calc = try!(input.parse_nested_block(CalcLengthOrPercentage::parse_length_or_percentage));
                 Ok(LengthOrPercentageOrNone::Calc(Box::new(calc)))
@@ -1055,7 +1134,7 @@ pub type LengthOrAuto = Either<Length, Auto>;
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub enum LengthOrPercentageOrAutoOrContent {
     /// A `<length>`.
-    Length(Length),
+    Length(NoCalcLength),
     /// A percentage.
     Percentage(Percentage),
     /// A `calc` node.
@@ -1093,11 +1172,11 @@ impl Parse for LengthOrPercentageOrAutoOrContent {
         let context = AllowedNumericType::NonNegative;
         match try!(input.next()) {
             Token::Dimension(ref value, ref unit) if context.is_ok(value.value) =>
-                Length::parse_dimension(value.value, unit).map(LengthOrPercentageOrAutoOrContent::Length),
+                NoCalcLength::parse_dimension(value.value, unit).map(LengthOrPercentageOrAutoOrContent::Length),
             Token::Percentage(ref value) if context.is_ok(value.unit_value) =>
                 Ok(LengthOrPercentageOrAutoOrContent::Percentage(Percentage(value.unit_value))),
             Token::Number(ref value) if value.value == 0. =>
-                Ok(LengthOrPercentageOrAutoOrContent::Length(Length::Absolute(Au(0)))),
+                Ok(LengthOrPercentageOrAutoOrContent::Length(NoCalcLength::zero())),
             Token::Ident(ref value) if value.eq_ignore_ascii_case("auto") =>
                 Ok(LengthOrPercentageOrAutoOrContent::Auto),
             Token::Ident(ref value) if value.eq_ignore_ascii_case("content") =>
diff --git a/components/style/values/specified/mod.rs b/components/style/values/specified/mod.rs
index 1a5b9807405..076324cebe6 100644
--- a/components/style/values/specified/mod.rs
+++ b/components/style/values/specified/mod.rs
@@ -26,7 +26,7 @@ pub use self::image::{GradientKind, HorizontalDirection, Image, LengthOrKeyword,
 pub use self::image::{SizeKeyword, VerticalDirection};
 pub use self::length::{FontRelativeLength, ViewportPercentageLength, CharacterWidth, Length, CalcLengthOrPercentage};
 pub use self::length::{Percentage, LengthOrNone, LengthOrNumber, LengthOrPercentage, LengthOrPercentageOrAuto};
-pub use self::length::{LengthOrPercentageOrNone, LengthOrPercentageOrAutoOrContent, CalcUnit};
+pub use self::length::{LengthOrPercentageOrNone, LengthOrPercentageOrAutoOrContent, NoCalcLength, CalcUnit};
 
 pub mod basic_shape;
 pub mod grid;
@@ -109,13 +109,14 @@ impl<'a> Mul<CSSFloat> for &'a SimplifiedSumNode {
 #[derive(Clone, Debug)]
 #[allow(missing_docs)]
 pub enum SimplifiedValueNode {
-    Length(Length),
+    Length(NoCalcLength),
     Angle(Angle),
     Time(Time),
     Percentage(CSSFloat),
     Number(CSSFloat),
     Sum(Box<SimplifiedSumNode>),
 }
+
 impl<'a> Mul<CSSFloat> for &'a SimplifiedValueNode {
     type Output = SimplifiedValueNode;
 
@@ -197,8 +198,8 @@ impl NoViewportPercentage for BorderRadiusSize {}
 impl BorderRadiusSize {
     #[allow(missing_docs)]
     pub fn zero() -> BorderRadiusSize {
-        let zero = LengthOrPercentage::Length(Length::Absolute(Au(0)));
-            BorderRadiusSize(Size2D::new(zero.clone(), zero))
+        let zero = LengthOrPercentage::Length(NoCalcLength::zero());
+        BorderRadiusSize(Size2D::new(zero.clone(), zero))
     }
 
     #[allow(missing_docs)]
@@ -291,11 +292,11 @@ pub fn parse_border_radius(context: &ParserContext, input: &mut Parser) -> Resul
     input.try(|i| BorderRadiusSize::parse(context, i)).or_else(|_| {
         match_ignore_ascii_case! { try!(input.expect_ident()),
             "thin" => Ok(BorderRadiusSize::circle(
-                             LengthOrPercentage::Length(Length::from_px(1.)))),
+                             LengthOrPercentage::Length(NoCalcLength::from_px(1.)))),
             "medium" => Ok(BorderRadiusSize::circle(
-                               LengthOrPercentage::Length(Length::from_px(3.)))),
+                               LengthOrPercentage::Length(NoCalcLength::from_px(3.)))),
             "thick" => Ok(BorderRadiusSize::circle(
-                              LengthOrPercentage::Length(Length::from_px(5.)))),
+                              LengthOrPercentage::Length(NoCalcLength::from_px(5.)))),
             _ => Err(())
         }
     })
@@ -604,12 +605,8 @@ impl Shadow {
     // disable_spread_and_inset is for filter: drop-shadow(...)
     #[allow(missing_docs)]
     pub fn parse(context:  &ParserContext, input: &mut Parser, disable_spread_and_inset: bool) -> Result<Shadow, ()> {
-        use app_units::Au;
         let length_count = if disable_spread_and_inset { 3 } else { 4 };
-        let mut lengths = [Length::Absolute(Au(0)),
-                           Length::Absolute(Au(0)),
-                           Length::Absolute(Au(0)),
-                           Length::Absolute(Au(0))];
+        let mut lengths = [Length::zero(), Length::zero(), Length::zero(), Length::zero()];
         let mut lengths_parsed = false;
         let mut color = None;
         let mut inset = false;
@@ -661,7 +658,7 @@ impl Shadow {
             offset_x: lengths[0].take(),
             offset_y: lengths[1].take(),
             blur_radius: lengths[2].take(),
-            spread_radius: if disable_spread_and_inset { Length::Absolute(Au(0)) } else { lengths[3].take() },
+            spread_radius: if disable_spread_and_inset { Length::zero() } else { lengths[3].take() },
             color: color,
             inset: inset,
         })
diff --git a/components/style/viewport.rs b/components/style/viewport.rs
index 7e98b7e2460..30ba3c0e7ab 100644
--- a/components/style/viewport.rs
+++ b/components/style/viewport.rs
@@ -25,7 +25,7 @@ use style_traits::ToCss;
 use style_traits::viewport::{Orientation, UserZoom, ViewportConstraints, Zoom};
 use stylesheets::{Stylesheet, Origin};
 use values::computed::{Context, ToComputedValue};
-use values::specified::{Length, LengthOrPercentageOrAuto, ViewportPercentageLength};
+use values::specified::{NoCalcLength, LengthOrPercentageOrAuto, ViewportPercentageLength};
 
 macro_rules! declare_viewport_descriptor {
     ( $( $variant_name: expr => $variant: ident($data: ident), )+ ) => {
@@ -150,14 +150,14 @@ impl FromMeta for ViewportLength {
 
         Some(match value {
             v if v.eq_ignore_ascii_case("device-width") =>
-                specified!(Length::ViewportPercentage(ViewportPercentageLength::Vw(100.))),
+                specified!(NoCalcLength::ViewportPercentage(ViewportPercentageLength::Vw(100.))),
             v if v.eq_ignore_ascii_case("device-height") =>
-                specified!(Length::ViewportPercentage(ViewportPercentageLength::Vh(100.))),
+                specified!(NoCalcLength::ViewportPercentage(ViewportPercentageLength::Vh(100.))),
             _ => {
                 match value.parse::<f32>() {
-                    Ok(n) if n >= 0. => specified!(Length::from_px(n.max(1.).min(10000.))),
+                    Ok(n) if n >= 0. => specified!(NoCalcLength::from_px(n.max(1.).min(10000.))),
                     Ok(_) => return None,
-                    Err(_) => specified!(Length::from_px(1.))
+                    Err(_) => specified!(NoCalcLength::from_px(1.))
                 }
             }
         })
diff --git a/tests/unit/style/media_queries.rs b/tests/unit/style/media_queries.rs
index d6e32a9e5a5..11e1987928a 100644
--- a/tests/unit/style/media_queries.rs
+++ b/tests/unit/style/media_queries.rs
@@ -2,7 +2,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use app_units::Au;
 use cssparser::{Parser, SourcePosition};
 use euclid::size::TypedSize2D;
 use servo_url::ServoUrl;
@@ -207,7 +206,7 @@ fn test_mq_default_expressions() {
         assert!(q.media_type == MediaQueryType::All, css.to_owned());
         assert!(q.expressions.len() == 1, css.to_owned());
         match *q.expressions[0].kind_for_testing() {
-            ExpressionKind::Width(Range::Min(ref w)) => assert!(*w == specified::Length::Absolute(Au::from_px(100))),
+            ExpressionKind::Width(Range::Min(ref w)) => assert!(*w == specified::Length::from_px(100.)),
             _ => panic!("wrong expression type"),
         }
     });
@@ -219,7 +218,7 @@ fn test_mq_default_expressions() {
         assert!(q.media_type == MediaQueryType::All, css.to_owned());
         assert!(q.expressions.len() == 1, css.to_owned());
         match *q.expressions[0].kind_for_testing() {
-            ExpressionKind::Width(Range::Max(ref w)) => assert!(*w == specified::Length::Absolute(Au::from_px(43))),
+            ExpressionKind::Width(Range::Max(ref w)) => assert!(*w == specified::Length::from_px(43.)),
             _ => panic!("wrong expression type"),
         }
     });
@@ -234,7 +233,7 @@ fn test_mq_expressions() {
         assert!(q.media_type == MediaQueryType::Known(MediaType::Screen), css.to_owned());
         assert!(q.expressions.len() == 1, css.to_owned());
         match *q.expressions[0].kind_for_testing() {
-            ExpressionKind::Width(Range::Min(ref w)) => assert!(*w == specified::Length::Absolute(Au::from_px(100))),
+            ExpressionKind::Width(Range::Min(ref w)) => assert!(*w == specified::Length::from_px(100.)),
             _ => panic!("wrong expression type"),
         }
     });
@@ -246,7 +245,7 @@ fn test_mq_expressions() {
         assert!(q.media_type == MediaQueryType::Known(MediaType::Print), css.to_owned());
         assert!(q.expressions.len() == 1, css.to_owned());
         match *q.expressions[0].kind_for_testing() {
-            ExpressionKind::Width(Range::Max(ref w)) => assert!(*w == specified::Length::Absolute(Au::from_px(43))),
+            ExpressionKind::Width(Range::Max(ref w)) => assert!(*w == specified::Length::from_px(43.)),
             _ => panic!("wrong expression type"),
         }
     });
@@ -258,7 +257,7 @@ fn test_mq_expressions() {
         assert!(q.media_type == MediaQueryType::Known(MediaType::Print), css.to_owned());
         assert!(q.expressions.len() == 1, css.to_owned());
         match *q.expressions[0].kind_for_testing() {
-            ExpressionKind::Width(Range::Eq(ref w)) => assert!(*w == specified::Length::Absolute(Au::from_px(43))),
+            ExpressionKind::Width(Range::Eq(ref w)) => assert!(*w == specified::Length::from_px(43.)),
             _ => panic!("wrong expression type"),
         }
     });
@@ -270,7 +269,7 @@ fn test_mq_expressions() {
         assert!(q.media_type == MediaQueryType::Unknown(Atom::from("fridge")), css.to_owned());
         assert!(q.expressions.len() == 1, css.to_owned());
         match *q.expressions[0].kind_for_testing() {
-            ExpressionKind::Width(Range::Max(ref w)) => assert!(*w == specified::Length::Absolute(Au::from_px(52))),
+            ExpressionKind::Width(Range::Max(ref w)) => assert!(*w == specified::Length::from_px(52.)),
             _ => panic!("wrong expression type"),
         }
     });
@@ -295,11 +294,11 @@ fn test_mq_multiple_expressions() {
         assert!(q.media_type == MediaQueryType::All, css.to_owned());
         assert!(q.expressions.len() == 2, css.to_owned());
         match *q.expressions[0].kind_for_testing() {
-            ExpressionKind::Width(Range::Min(ref w)) => assert!(*w == specified::Length::Absolute(Au::from_px(100))),
+            ExpressionKind::Width(Range::Min(ref w)) => assert!(*w == specified::Length::from_px(100.)),
             _ => panic!("wrong expression type"),
         }
         match *q.expressions[1].kind_for_testing() {
-            ExpressionKind::Width(Range::Max(ref w)) => assert!(*w == specified::Length::Absolute(Au::from_px(200))),
+            ExpressionKind::Width(Range::Max(ref w)) => assert!(*w == specified::Length::from_px(200.)),
             _ => panic!("wrong expression type"),
         }
     });
@@ -311,11 +310,11 @@ fn test_mq_multiple_expressions() {
         assert!(q.media_type == MediaQueryType::Known(MediaType::Screen), css.to_owned());
         assert!(q.expressions.len() == 2, css.to_owned());
         match *q.expressions[0].kind_for_testing() {
-            ExpressionKind::Width(Range::Min(ref w)) => assert!(*w == specified::Length::Absolute(Au::from_px(100))),
+            ExpressionKind::Width(Range::Min(ref w)) => assert!(*w == specified::Length::from_px(100.)),
             _ => panic!("wrong expression type"),
         }
         match *q.expressions[1].kind_for_testing() {
-            ExpressionKind::Width(Range::Max(ref w)) => assert!(*w == specified::Length::Absolute(Au::from_px(200))),
+            ExpressionKind::Width(Range::Max(ref w)) => assert!(*w == specified::Length::from_px(200.)),
             _ => panic!("wrong expression type"),
         }
     });
diff --git a/tests/unit/style/properties/serialization.rs b/tests/unit/style/properties/serialization.rs
index ec608fa6ccb..4d83aad5905 100644
--- a/tests/unit/style/properties/serialization.rs
+++ b/tests/unit/style/properties/serialization.rs
@@ -5,7 +5,7 @@
 pub use std::sync::Arc;
 pub use style::computed_values::display::T::inline_block;
 pub use style::properties::{DeclaredValue, PropertyDeclaration, PropertyDeclarationBlock, Importance, PropertyId};
-pub use style::values::specified::{BorderStyle, BorderWidth, CSSColor, Length};
+pub use style::values::specified::{BorderStyle, BorderWidth, CSSColor, Length, NoCalcLength};
 pub use style::values::specified::{LengthOrPercentage, LengthOrPercentageOrAuto, LengthOrPercentageOrAutoOrContent};
 pub use style::properties::longhands::outline_color::computed_value::T as ComputedColor;
 pub use style::values::RGBA;
@@ -19,15 +19,15 @@ fn property_declaration_block_should_serialize_correctly() {
 
     let declarations = vec![
         (PropertyDeclaration::Width(
-            DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(70f32)))),
+            DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(70f32)))),
          Importance::Normal),
 
         (PropertyDeclaration::MinHeight(
-            DeclaredValue::Value(LengthOrPercentage::Length(Length::from_px(20f32)))),
+            DeclaredValue::Value(LengthOrPercentage::Length(NoCalcLength::from_px(20f32)))),
          Importance::Normal),
 
         (PropertyDeclaration::Height(
-            DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(20f32)))),
+            DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(20f32)))),
          Importance::Important),
 
         (PropertyDeclaration::Display(
@@ -113,7 +113,7 @@ mod shorthand_serialization {
         fn all_equal_properties_should_serialize_to_one_value() {
             let mut properties = Vec::new();
 
-            let px_70 = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(70f32)));
+            let px_70 = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(70f32)));
             properties.push(PropertyDeclaration::MarginTop(px_70.clone()));
             properties.push(PropertyDeclaration::MarginRight(px_70.clone()));
             properties.push(PropertyDeclaration::MarginBottom(px_70.clone()));
@@ -127,8 +127,8 @@ mod shorthand_serialization {
         fn equal_vertical_and_equal_horizontal_properties_should_serialize_to_two_value() {
             let mut properties = Vec::new();
 
-            let vertical_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(10f32)));
-            let horizontal_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(5f32)));
+            let vertical_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(10f32)));
+            let horizontal_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(5f32)));
 
             properties.push(PropertyDeclaration::MarginTop(vertical_px.clone()));
             properties.push(PropertyDeclaration::MarginRight(horizontal_px.clone()));
@@ -143,9 +143,9 @@ mod shorthand_serialization {
         fn different_vertical_and_equal_horizontal_properties_should_serialize_to_three_values() {
             let mut properties = Vec::new();
 
-            let top_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(8f32)));
-            let bottom_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(10f32)));
-            let horizontal_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(5f32)));
+            let top_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(8f32)));
+            let bottom_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(10f32)));
+            let horizontal_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(5f32)));
 
             properties.push(PropertyDeclaration::MarginTop(top_px));
             properties.push(PropertyDeclaration::MarginRight(horizontal_px.clone()));
@@ -160,10 +160,10 @@ mod shorthand_serialization {
         fn different_properties_should_serialize_to_four_values() {
             let mut properties = Vec::new();
 
-            let top_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(8f32)));
-            let right_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(12f32)));
-            let bottom_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(10f32)));
-            let left_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(Length::from_px(14f32)));
+            let top_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(8f32)));
+            let right_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(12f32)));
+            let bottom_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(10f32)));
+            let left_px = DeclaredValue::Value(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(14f32)));
 
             properties.push(PropertyDeclaration::MarginTop(top_px));
             properties.push(PropertyDeclaration::MarginRight(right_px));
@@ -178,8 +178,8 @@ mod shorthand_serialization {
         fn padding_should_serialize_correctly() {
             let mut properties = Vec::new();
 
-            let px_10 = DeclaredValue::Value(LengthOrPercentage::Length(Length::from_px(10f32)));
-            let px_15 = DeclaredValue::Value(LengthOrPercentage::Length(Length::from_px(15f32)));
+            let px_10 = DeclaredValue::Value(LengthOrPercentage::Length(NoCalcLength::from_px(10f32)));
+            let px_15 = DeclaredValue::Value(LengthOrPercentage::Length(NoCalcLength::from_px(15f32)));
             properties.push(PropertyDeclaration::PaddingTop(px_10.clone()));
             properties.push(PropertyDeclaration::PaddingRight(px_15.clone()));
             properties.push(PropertyDeclaration::PaddingBottom(px_10));
@@ -637,7 +637,7 @@ mod shorthand_serialization {
             let font_variant = DeclaredValue::Value(FontVariant::normal);
             let font_weight = DeclaredValue::Value(FontWeight::Bolder);
             let font_size = DeclaredValue::Value(FontSizeContainer(
-                LengthOrPercentage::Length(Length::from_px(4f32)))
+                LengthOrPercentage::Length(NoCalcLength::from_px(4f32)))
             );
             let font_stretch = DeclaredValue::Value(FontStretch::expanded);
             let line_height = DeclaredValue::Value(LineHeight::Number(3f32));
@@ -718,14 +718,14 @@ mod shorthand_serialization {
             let position_x = single_vec_value_typedef!(position_x,
                 HorizontalPosition {
                     keyword: None,
-                    position: Some(LengthOrPercentage::Length(Length::from_px(7f32))),
+                    position: Some(LengthOrPercentage::Length(NoCalcLength::from_px(7f32))),
                 }
             );
 
             let position_y = single_vec_value_typedef!(position_y,
                 VerticalPosition {
                     keyword: None,
-                    position: Some(LengthOrPercentage::Length(Length::from_px(4f32))),
+                    position: Some(LengthOrPercentage::Length(NoCalcLength::from_px(4f32))),
                 }
             );
 
@@ -738,8 +738,8 @@ mod shorthand_serialization {
             let size = single_vec_variant_value!(size,
                 size::single_value::SpecifiedValue::Explicit(
                     size::single_value::ExplicitSize {
-                        width: LengthOrPercentageOrAuto::Length(Length::from_px(70f32)),
-                        height: LengthOrPercentageOrAuto::Length(Length::from_px(50f32))
+                        width: LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(70f32)),
+                        height: LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(50f32))
                     }
                 )
             );
@@ -778,14 +778,14 @@ mod shorthand_serialization {
             let position_x = single_vec_value_typedef!(position_x,
                 HorizontalPosition {
                     keyword: None,
-                    position: Some(LengthOrPercentage::Length(Length::from_px(7f32))),
+                    position: Some(LengthOrPercentage::Length(NoCalcLength::from_px(7f32))),
                 }
             );
 
             let position_y = single_vec_value_typedef!(position_y,
                 VerticalPosition {
                     keyword: None,
-                    position: Some(LengthOrPercentage::Length(Length::from_px(4f32))),
+                    position: Some(LengthOrPercentage::Length(NoCalcLength::from_px(4f32))),
                 }
             );
 
@@ -798,8 +798,8 @@ mod shorthand_serialization {
             let size = single_vec_variant_value!(size,
                 size::single_value::SpecifiedValue::Explicit(
                     size::single_value::ExplicitSize {
-                        width: LengthOrPercentageOrAuto::Length(Length::from_px(70f32)),
-                        height: LengthOrPercentageOrAuto::Length(Length::from_px(50f32))
+                        width: LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(70f32)),
+                        height: LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(50f32))
                     }
                 )
             );
@@ -837,14 +837,14 @@ mod shorthand_serialization {
             let position_x = single_vec_value_typedef!(position_x,
                 HorizontalPosition {
                     keyword: None,
-                    position: Some(LengthOrPercentage::Length(Length::from_px(0f32))),
+                    position: Some(LengthOrPercentage::Length(NoCalcLength::from_px(0f32))),
                 }
             );
 
             let position_y = single_vec_value_typedef!(position_y,
                 VerticalPosition {
                     keyword: None,
-                    position: Some(LengthOrPercentage::Length(Length::from_px(0f32))),
+                    position: Some(LengthOrPercentage::Length(NoCalcLength::from_px(0f32))),
                 }
             );
 
@@ -922,11 +922,11 @@ mod shorthand_serialization {
                 Position {
                     horizontal: HorizontalPosition {
                         keyword: None,
-                        position: Some(LengthOrPercentage::Length(Length::from_px(7f32))),
+                        position: Some(LengthOrPercentage::Length(NoCalcLength::from_px(7f32))),
                     },
                     vertical: VerticalPosition {
                         keyword: None,
-                        position: Some(LengthOrPercentage::Length(Length::from_px(4f32))),
+                        position: Some(LengthOrPercentage::Length(NoCalcLength::from_px(4f32))),
                     },
                 }
             );
@@ -934,8 +934,8 @@ mod shorthand_serialization {
             let size = single_vec_variant_value!(size,
                 size::single_value::SpecifiedValue::Explicit(
                     size::single_value::ExplicitSize {
-                        width: LengthOrPercentageOrAuto::Length(Length::from_px(70f32)),
-                        height: LengthOrPercentageOrAuto::Length(Length::from_px(50f32))
+                        width: LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(70f32)),
+                        height: LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(50f32))
                     }
                 )
             );
@@ -976,11 +976,11 @@ mod shorthand_serialization {
                 Position {
                     horizontal: HorizontalPosition {
                         keyword: None,
-                        position: Some(LengthOrPercentage::Length(Length::from_px(7f32))),
+                        position: Some(LengthOrPercentage::Length(NoCalcLength::from_px(7f32))),
                     },
                     vertical: VerticalPosition {
                         keyword: None,
-                        position: Some(LengthOrPercentage::Length(Length::from_px(4f32))),
+                        position: Some(LengthOrPercentage::Length(NoCalcLength::from_px(4f32))),
                     },
                 }
             );
@@ -988,8 +988,8 @@ mod shorthand_serialization {
             let size = single_vec_variant_value!(size,
                 size::single_value::SpecifiedValue::Explicit(
                     size::single_value::ExplicitSize {
-                        width: LengthOrPercentageOrAuto::Length(Length::from_px(70f32)),
-                        height: LengthOrPercentageOrAuto::Length(Length::from_px(50f32))
+                        width: LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(70f32)),
+                        height: LengthOrPercentageOrAuto::Length(NoCalcLength::from_px(50f32))
                     }
                 )
             );
diff --git a/tests/unit/style/properties/viewport.rs b/tests/unit/style/properties/viewport.rs
index 221e7d568bb..7e036a6e95d 100644
--- a/tests/unit/style/properties/viewport.rs
+++ b/tests/unit/style/properties/viewport.rs
@@ -6,21 +6,21 @@ use app_units::Au;
 use style::properties::{DeclaredValue, PropertyDeclaration};
 use style::properties::longhands::border_top_width;
 use style::values::HasViewportPercentage;
-use style::values::specified::{Length, ViewportPercentageLength};
+use style::values::specified::{Length, NoCalcLength, ViewportPercentageLength};
 
 #[test]
 fn has_viewport_percentage_for_specified_value() {
     //TODO: test all specified value with a HasViewportPercentage impl
     let pvw = PropertyDeclaration::BorderTopWidth(
         DeclaredValue::Value(border_top_width::SpecifiedValue::from_length(
-            Length::ViewportPercentage(ViewportPercentageLength::Vw(100.))
+            Length::NoCalc(NoCalcLength::ViewportPercentage(ViewportPercentageLength::Vw(100.)))
         ))
     );
     assert!(pvw.has_viewport_percentage());
 
     let pabs = PropertyDeclaration::BorderTopWidth(
         DeclaredValue::Value(border_top_width::SpecifiedValue::from_length(
-            Length::Absolute(Au(100))
+            Length::NoCalc(NoCalcLength::Absolute(Au(100)))
         ))
     );
     assert!(!pabs.has_viewport_percentage());
diff --git a/tests/unit/style/value.rs b/tests/unit/style/value.rs
index 4f9944e324d..472a5177d30 100644
--- a/tests/unit/style/value.rs
+++ b/tests/unit/style/value.rs
@@ -5,14 +5,14 @@
 use app_units::Au;
 use cssparser::Parser;
 use style::values::HasViewportPercentage;
-use style::values::specified::{ViewportPercentageLength, Length};
+use style::values::specified::{ViewportPercentageLength, NoCalcLength};
 use style::values::specified::length::{CalcLengthOrPercentage, CalcUnit};
 
 #[test]
 fn length_has_viewport_percentage() {
-    let l = Length::ViewportPercentage(ViewportPercentageLength::Vw(100.));
+    let l = NoCalcLength::ViewportPercentage(ViewportPercentageLength::Vw(100.));
     assert!(l.has_viewport_percentage());
-    let l = Length::Absolute(Au(100));
+    let l = NoCalcLength::Absolute(Au(100));
     assert!(!l.has_viewport_percentage());
 }
 
diff --git a/tests/unit/style/viewport.rs b/tests/unit/style/viewport.rs
index dce0deafca8..d01a00363bf 100644
--- a/tests/unit/style/viewport.rs
+++ b/tests/unit/style/viewport.rs
@@ -12,8 +12,8 @@ use style::error_reporting::ParseErrorReporter;
 use style::media_queries::{Device, MediaType};
 use style::parser::{ParserContext, ParserContextExtraData};
 use style::stylesheets::{Stylesheet, Origin};
-use style::values::specified::Length::{self, ViewportPercentage};
 use style::values::specified::LengthOrPercentageOrAuto::{self, Auto};
+use style::values::specified::NoCalcLength::{self, ViewportPercentage};
 use style::values::specified::ViewportPercentageLength::Vw;
 use style::viewport::*;
 use style_traits::viewport::*;
@@ -80,7 +80,7 @@ macro_rules! assert_decl_len {
 
 macro_rules! viewport_length {
     ($value:expr, px) => {
-        ViewportLength::Specified(LengthOrPercentageOrAuto::Length(Length::from_px($value)))
+        ViewportLength::Specified(LengthOrPercentageOrAuto::Length(NoCalcLength::from_px($value)))
     };
     ($value:expr, vw) => {
         ViewportLength::Specified(LengthOrPercentageOrAuto::Length(ViewportPercentage(Vw($value))))