Support unit variants when deriving ToCss

14 files changed, 71 insertions, 242 deletions

diff --git a/components/style/properties/longhand/box.mako.rs b/components/style/properties/longhand/box.mako.rs
index 623810c8724..59c1e669aea 100644
--- a/components/style/properties/longhand/box.mako.rs
+++ b/components/style/properties/longhand/box.mako.rs
@@ -545,8 +545,6 @@ ${helpers.predefined_type("animation-timing-function",
                           extra_prefixes="moz webkit"
                           spec="https://drafts.csswg.org/css-animations/#propdef-animation-iteration-count",
                           allowed_in_keyframe_block="False">
-    use std::fmt;
-    use style_traits::ToCss;
     use values::computed::ComputedValueAsSpecified;
 
     pub mod computed_value {
@@ -554,8 +552,8 @@ ${helpers.predefined_type("animation-timing-function",
     }
 
     // https://drafts.csswg.org/css-animations/#animation-iteration-count
-    #[derive(Debug, Clone, PartialEq)]
     #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+    #[derive(Debug, Clone, PartialEq, ToCss)]
     pub enum SpecifiedValue {
         Number(f32),
         Infinite,
@@ -576,15 +574,6 @@ ${helpers.predefined_type("animation-timing-function",
         }
     }
 
-    impl ToCss for SpecifiedValue {
-        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            match *self {
-                SpecifiedValue::Number(n) => write!(dest, "{}", n),
-                SpecifiedValue::Infinite => dest.write_str("infinite"),
-            }
-        }
-    }
-
     no_viewport_percentage!(SpecifiedValue);
 
     #[inline]
diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index 2c491a1630a..51963a7dbb0 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -1039,28 +1039,15 @@ ${helpers.single_keyword_system("font-variant-caps",
 
     pub mod computed_value {
         use properties::animated_properties::Animatable;
-        use std::fmt;
-        use style_traits::ToCss;
         use values::CSSFloat;
 
-        #[derive(Copy, Clone, Debug, PartialEq)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+        #[derive(Copy, Clone, Debug, PartialEq, ToCss)]
         pub enum T {
             None,
             Number(CSSFloat),
         }
 
-        impl ToCss for T {
-            fn to_css<W>(&self, dest: &mut W) -> fmt::Result
-                where W: fmt::Write,
-            {
-                match *self {
-                    T::None => dest.write_str("none"),
-                    T::Number(number) => number.to_css(dest),
-                }
-            }
-        }
-
         impl T {
             pub fn from_gecko_adjust(gecko: f32) -> Self {
                 if gecko == -1.0 {
@@ -2214,9 +2201,7 @@ ${helpers.single_keyword("-moz-math-variant",
         use app_units::Au;
         use cssparser::Parser;
         use properties::longhands;
-        use std::fmt;
         use std::hash::{Hash, Hasher};
-        use style_traits::ToCss;
         use values::computed::{ToComputedValue, Context};
         <%
             system_fonts = """caption icon menu message-box small-caption status-bar
@@ -2230,23 +2215,13 @@ ${helpers.single_keyword("-moz-math-variant",
             kw_cast = """font_style font_variant_caps font_stretch
                          font_kerning font_variant_position""".split()
         %>
-        #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+        #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq, ToCss)]
         pub enum SystemFont {
             % for font in system_fonts:
                 ${to_camel_case(font)},
             % endfor
         }
 
-        impl ToCss for SystemFont {
-            fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-                dest.write_str(match *self {
-                    % for font in system_fonts:
-                        SystemFont::${to_camel_case(font)} => "${font}",
-                    % endfor
-                })
-            }
-        }
-
         // ComputedValues are compared at times
         // so we need these impls. We don't want to
         // add Eq to Number (which contains a float)
diff --git a/components/style/properties/properties.mako.rs b/components/style/properties/properties.mako.rs
index 4114b2f2b22..7c0ea1e9abb 100644
--- a/components/style/properties/properties.mako.rs
+++ b/components/style/properties/properties.mako.rs
@@ -450,8 +450,8 @@ impl PropertyDeclarationIdSet {
 % endfor
 
 /// An enum to represent a CSS Wide keyword.
-#[derive(Copy, Clone, PartialEq, Eq, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Copy, Clone, Debug, Eq, PartialEq, ToCss)]
 pub enum CSSWideKeyword {
     /// The `initial` keyword.
     Initial,
@@ -483,12 +483,6 @@ impl CSSWideKeyword {
     }
 }
 
-impl ToCss for CSSWideKeyword {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        dest.write_str(self.to_str())
-    }
-}
-
 impl Parse for CSSWideKeyword {
     fn parse(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
         let ident = input.expect_ident()?;
@@ -628,8 +622,8 @@ impl LonghandId {
 }
 
 /// An identifier for a given shorthand property.
-#[derive(Clone, Copy, Eq, PartialEq, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, Eq, PartialEq, ToCss)]
 pub enum ShorthandId {
     % for property in data.shorthands:
         /// ${property.name}
@@ -637,14 +631,6 @@ pub enum ShorthandId {
     % endfor
 }
 
-impl ToCss for ShorthandId {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
-        where W: fmt::Write,
-    {
-        dest.write_str(self.name())
-    }
-}
-
 impl ShorthandId {
     /// Get the name for this shorthand property.
     pub fn name(&self) -> &'static str {
diff --git a/components/style/values/generics/background.rs b/components/style/values/generics/background.rs
index af8cef23a38..aa24454ef6d 100644
--- a/components/style/values/generics/background.rs
+++ b/components/style/values/generics/background.rs
@@ -4,11 +4,8 @@
 
 //! Generic types for CSS values related to backgrounds.
 
-use std::fmt;
-use style_traits::ToCss;
-
 /// A generic value for the `background-size` property.
-#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
+#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue, ToCss)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub enum BackgroundSize<LengthOrPercentageOrAuto> {
     /// `<width> <height>`
@@ -32,21 +29,3 @@ impl<L> From<L> for BackgroundSize<L>
         BackgroundSize::Explicit { width: value.clone(), height: value }
     }
 }
-
-impl<L> ToCss for BackgroundSize<L>
-    where L: ToCss
-{
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
-        where W: fmt::Write
-    {
-        match *self {
-            BackgroundSize::Explicit { ref width, ref height } => {
-                width.to_css(dest)?;
-                dest.write_str(" ")?;
-                height.to_css(dest)
-            },
-            BackgroundSize::Cover => dest.write_str("cover"),
-            BackgroundSize::Contain => dest.write_str("contain"),
-        }
-    }
-}
diff --git a/components/style/values/generics/basic_shape.rs b/components/style/values/generics/basic_shape.rs
index b2d402dd38f..cd8e315f3b3 100644
--- a/components/style/values/generics/basic_shape.rs
+++ b/components/style/values/generics/basic_shape.rs
@@ -19,7 +19,7 @@ pub type ClippingShape<BasicShape> = ShapeSource<BasicShape, GeometryBox>;
 /// https://drafts.fxtf.org/css-masking-1/#typedef-geometry-box
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, Debug, PartialEq)]
+#[derive(Clone, Copy, Debug, PartialEq, ToCss)]
 pub enum GeometryBox {
     FillBox,
     StrokeBox,
@@ -142,17 +142,6 @@ impl<B: ToCss, T: ToCss> ToCss for ShapeSource<B, T> {
     }
 }
 
-impl ToCss for GeometryBox {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            GeometryBox::FillBox => dest.write_str("fill-box"),
-            GeometryBox::StrokeBox => dest.write_str("stroke-box"),
-            GeometryBox::ViewBox => dest.write_str("view-box"),
-            GeometryBox::ShapeBox(s) => s.to_css(dest),
-        }
-    }
-}
-
 impl<L> ToCss for InsetRect<L>
     where L: ToCss + PartialEq
 {
diff --git a/components/style/values/generics/border.rs b/components/style/values/generics/border.rs
index f1384e580e3..5d8c9032e6a 100644
--- a/components/style/values/generics/border.rs
+++ b/components/style/values/generics/border.rs
@@ -11,7 +11,7 @@ use values::generics::rect::Rect;
 
 /// A generic value for a single side of a `border-image-width` property.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
+#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue, ToCss)]
 pub enum BorderImageSideWidth<LengthOrPercentage, Number> {
     /// `<length-or-percentage>`
     Length(LengthOrPercentage),
@@ -52,20 +52,6 @@ pub struct BorderRadius<LengthOrPercentage> {
 /// A generic value for `border-*-radius` longhand properties.
 pub struct BorderCornerRadius<L>(pub Size2D<L>);
 
-impl<L, N> ToCss for BorderImageSideWidth<L, N>
-    where L: ToCss, N: ToCss,
-{
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
-        where W: fmt::Write
-    {
-        match *self {
-            BorderImageSideWidth::Length(ref length) => length.to_css(dest),
-            BorderImageSideWidth::Number(ref number) => number.to_css(dest),
-            BorderImageSideWidth::Auto => dest.write_str("auto"),
-        }
-    }
-}
-
 impl<N> From<N> for BorderImageSlice<N>
     where N: Clone,
 {
diff --git a/components/style/values/generics/grid.rs b/components/style/values/generics/grid.rs
index 73640a91e49..3775bb30597 100644
--- a/components/style/values/generics/grid.rs
+++ b/components/style/values/generics/grid.rs
@@ -329,8 +329,8 @@ pub fn concat_serialize_idents<W>(prefix: &str, suffix: &str,
 /// The initial argument of the `repeat` function.
 ///
 /// https://drafts.csswg.org/css-grid/#typedef-track-repeat
-#[derive(Clone, Copy, PartialEq, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, PartialEq, ToCss)]
 pub enum RepeatCount {
     /// A positive integer. This is allowed only for `<track-repeat>` and `<fixed-repeat>`
     Number(Integer),
@@ -340,16 +340,6 @@ pub enum RepeatCount {
     AutoFit,
 }
 
-impl ToCss for RepeatCount {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            RepeatCount::Number(ref c) => c.to_css(dest),
-            RepeatCount::AutoFill => dest.write_str("auto-fill"),
-            RepeatCount::AutoFit => dest.write_str("auto-fit"),
-        }
-    }
-}
-
 impl Parse for RepeatCount {
     fn parse(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
         if let Ok(i) = input.try(|i| Integer::parse(context, i)) {
diff --git a/components/style/values/generics/mod.rs b/components/style/values/generics/mod.rs
index ef88d7c342c..40d3f4e46df 100644
--- a/components/style/values/generics/mod.rs
+++ b/components/style/values/generics/mod.rs
@@ -182,8 +182,8 @@ impl<T: Parse> Parse for FontSettingTag<T> {
 
 
 /// A font settings value for font-variation-settings or font-feature-settings
-#[derive(Clone, Debug, Eq, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, Eq, PartialEq, ToCss)]
 pub enum FontSettings<T> {
     /// No settings (default)
     Normal,
@@ -191,15 +191,6 @@ pub enum FontSettings<T> {
     Tag(Vec<FontSettingTag<T>>)
 }
 
-impl<T: ToCss> ToCss for FontSettings<T> {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            FontSettings::Normal => dest.write_str("normal"),
-            FontSettings::Tag(ref ftvs) => ftvs.to_css(dest)
-        }
-    }
-}
-
 impl<T: Parse> Parse for FontSettings<T> {
     /// https://www.w3.org/TR/css-fonts-3/#propdef-font-feature-settings
     fn parse(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
@@ -290,8 +281,8 @@ pub struct SVGPaint<ColorType> {
 /// Whereas the spec only allows PaintServer
 /// to have a fallback, Gecko lets the context
 /// properties have a fallback as well.
-#[derive(Debug, Clone, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, PartialEq, ToCss)]
 pub enum SVGPaintKind<ColorType> {
     /// `none`
     None,
@@ -378,18 +369,6 @@ impl<ColorType: Parse> Parse for SVGPaint<ColorType> {
     }
 }
 
-impl<ColorType: ToCss> ToCss for SVGPaintKind<ColorType> {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            SVGPaintKind::None => dest.write_str("none"),
-            SVGPaintKind::ContextStroke => dest.write_str("context-stroke"),
-            SVGPaintKind::ContextFill => dest.write_str("context-fill"),
-            SVGPaintKind::Color(ref color) => color.to_css(dest),
-            SVGPaintKind::PaintServer(ref server) => server.to_css(dest),
-        }
-    }
-}
-
 impl<ColorType: ToCss> ToCss for SVGPaint<ColorType> {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
         self.kind.to_css(dest)?;
diff --git a/components/style/values/generics/text.rs b/components/style/values/generics/text.rs
index 0652fa91224..5a5a10bb9d9 100644
--- a/components/style/values/generics/text.rs
+++ b/components/style/values/generics/text.rs
@@ -54,7 +54,7 @@ where
 
 /// A generic spacing value for the `letter-spacing` and `word-spacing` properties.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
+#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue, ToCss)]
 pub enum Spacing<Value> {
     /// `normal`
     Normal,
@@ -117,22 +117,9 @@ impl<Value> Animatable for Spacing<Value>
     }
 }
 
-impl<Value> ToCss for Spacing<Value>
-    where Value: ToCss,
-{
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
-        where W: fmt::Write
-    {
-        match *self {
-            Spacing::Normal => dest.write_str("normal"),
-            Spacing::Value(ref value) => value.to_css(dest),
-        }
-    }
-}
-
 /// A generic value for the `line-height` property.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq)]
+#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToCss)]
 pub enum LineHeight<Number, LengthOrPercentage> {
     /// `normal`
     Normal,
@@ -152,19 +139,3 @@ impl<N, L> LineHeight<N, L> {
         LineHeight::Normal
     }
 }
-
-impl<N, L> ToCss for LineHeight<N, L>
-    where N: ToCss, L: ToCss,
-{
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
-        where W: fmt::Write,
-    {
-        match *self {
-            LineHeight::Normal => dest.write_str("normal"),
-            #[cfg(feature = "gecko")]
-            LineHeight::MozBlockHeight => dest.write_str("-moz-block-height"),
-            LineHeight::Number(ref number) => number.to_css(dest),
-            LineHeight::Length(ref value) => value.to_css(dest),
-        }
-    }
-}
diff --git a/components/style/values/specified/border.rs b/components/style/values/specified/border.rs
index b931ef36245..53260bc8e33 100644
--- a/components/style/values/specified/border.rs
+++ b/components/style/values/specified/border.rs
@@ -7,8 +7,6 @@
 use app_units::Au;
 use cssparser::Parser;
 use parser::{Parse, ParserContext};
-use std::fmt;
-use style_traits::ToCss;
 use values::computed::{Context, ToComputedValue};
 use values::generics::border::BorderCornerRadius as GenericBorderCornerRadius;
 use values::generics::border::BorderImageSideWidth as GenericBorderImageSideWidth;
@@ -20,7 +18,7 @@ use values::specified::length::{Length, LengthOrPercentage};
 
 /// A specified value for a single side of the `border-width` property.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
 pub enum BorderSideWidth {
     /// `thin`
     Thin,
@@ -73,17 +71,6 @@ impl Parse for BorderSideWidth {
     }
 }
 
-impl ToCss for BorderSideWidth {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            BorderSideWidth::Thin => dest.write_str("thin"),
-            BorderSideWidth::Medium => dest.write_str("medium"),
-            BorderSideWidth::Thick => dest.write_str("thick"),
-            BorderSideWidth::Length(ref length) => length.to_css(dest)
-        }
-    }
-}
-
 impl ToComputedValue for BorderSideWidth {
     type ComputedValue = Au;
 
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 61335ee0e55..e7b49ee9392 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -894,9 +894,9 @@ impl LengthOrPercentage {
 }
 
 /// Either a `<length>`, a `<percentage>`, or the `auto` keyword.
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
 pub enum LengthOrPercentageOrAuto {
     Length(NoCalcLength),
     Percentage(Percentage),
@@ -918,17 +918,6 @@ impl From<Percentage> for LengthOrPercentageOrAuto {
     }
 }
 
-impl ToCss for LengthOrPercentageOrAuto {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            LengthOrPercentageOrAuto::Length(ref length) => length.to_css(dest),
-            LengthOrPercentageOrAuto::Percentage(percentage) => percentage.to_css(dest),
-            LengthOrPercentageOrAuto::Auto => dest.write_str("auto"),
-            LengthOrPercentageOrAuto::Calc(ref calc) => calc.to_css(dest),
-        }
-    }
-}
-
 impl LengthOrPercentageOrAuto {
     fn parse_internal(context: &ParserContext,
                       input: &mut Parser,
@@ -1012,8 +1001,8 @@ impl LengthOrPercentageOrAuto {
 }
 
 /// Either a `<length>`, a `<percentage>`, or the `none` keyword.
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
 #[allow(missing_docs)]
 pub enum LengthOrPercentageOrNone {
     Length(NoCalcLength),
@@ -1022,16 +1011,6 @@ pub enum LengthOrPercentageOrNone {
     None,
 }
 
-impl ToCss for LengthOrPercentageOrNone {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            LengthOrPercentageOrNone::Length(ref length) => length.to_css(dest),
-            LengthOrPercentageOrNone::Percentage(ref percentage) => percentage.to_css(dest),
-            LengthOrPercentageOrNone::Calc(ref calc) => calc.to_css(dest),
-            LengthOrPercentageOrNone::None => dest.write_str("none"),
-        }
-    }
-}
 impl LengthOrPercentageOrNone {
     fn parse_internal(context: &ParserContext,
                       input: &mut Parser,
@@ -1099,8 +1078,8 @@ pub type LengthOrAuto = Either<Length, Auto>;
 
 /// Either a `<length>` or a `<percentage>` or the `auto` keyword or the
 /// `content` keyword.
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
 pub enum LengthOrPercentageOrAutoOrContent {
     /// A `<length>`.
     Length(NoCalcLength),
@@ -1156,18 +1135,6 @@ impl LengthOrPercentageOrAutoOrContent {
     }
 }
 
-impl ToCss for LengthOrPercentageOrAutoOrContent {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            LengthOrPercentageOrAutoOrContent::Length(ref len) => len.to_css(dest),
-            LengthOrPercentageOrAutoOrContent::Percentage(perc) => perc.to_css(dest),
-            LengthOrPercentageOrAutoOrContent::Auto => dest.write_str("auto"),
-            LengthOrPercentageOrAutoOrContent::Content => dest.write_str("content"),
-            LengthOrPercentageOrAutoOrContent::Calc(ref calc) => calc.to_css(dest),
-        }
-    }
-}
-
 /// Either a `<length>` or a `<number>`.
 pub type LengthOrNumber = Either<Length, Number>;
 
diff --git a/components/style/values/specified/transform.rs b/components/style/values/specified/transform.rs
index f41539fc5b2..e0e5b1c2d6f 100644
--- a/components/style/values/specified/transform.rs
+++ b/components/style/values/specified/transform.rs
@@ -7,8 +7,6 @@
 use cssparser::Parser;
 use euclid::Point2D;
 use parser::{Parse, ParserContext};
-use std::fmt;
-use style_traits::ToCss;
 use values::computed::{LengthOrPercentage as ComputedLengthOrPercentage, Context, ToComputedValue};
 use values::computed::transform::TimingFunction as ComputedTimingFunction;
 use values::generics::transform::{StepPosition, TimingFunction as GenericTimingFunction};
@@ -22,7 +20,7 @@ pub type TransformOrigin = GenericTransformOrigin<OriginComponent<X>, OriginComp
 
 /// The specified value of a component of a CSS `<transform-origin>`.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
 pub enum OriginComponent<S> {
     /// `center`
     Center,
@@ -99,20 +97,6 @@ impl<S> Parse for OriginComponent<S>
     }
 }
 
-impl<S: ToCss> ToCss for OriginComponent<S>
-    where S: ToCss,
-{
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
-        where W: fmt::Write,
-    {
-        match *self {
-            OriginComponent::Center => dest.write_str("center"),
-            OriginComponent::Length(ref lop) => lop.to_css(dest),
-            OriginComponent::Side(ref keyword) => keyword.to_css(dest),
-        }
-    }
-}
-
 impl<S> ToComputedValue for OriginComponent<S>
     where S: Side,
 {
diff --git a/components/style_derive/to_css.rs b/components/style_derive/to_css.rs
index 774add083fa..f85fe1ce418 100644
--- a/components/style_derive/to_css.rs
+++ b/components/style_derive/to_css.rs
@@ -15,9 +15,12 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
     }
 
     let style = synstructure::BindStyle::Ref.into();
-    let match_body = synstructure::each_variant(&input, &style, |bindings, _| {
+    let match_body = synstructure::each_variant(&input, &style, |bindings, variant| {
         if bindings.is_empty() {
-            panic!("unit variants are not yet supported");
+            let identifier = to_css_identifier(variant.ident.as_ref());
+            return Some(quote! {
+                ::std::fmt::Write::write_str(dest, #identifier)
+            });
         }
         let (first, rest) = bindings.split_first().expect("unit variants are not yet supported");
         where_clause.predicates.push(where_predicate(first.field.ty.clone()));
@@ -28,7 +31,7 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
             where_clause.predicates.push(where_predicate(binding.field.ty.clone()));
             expr = quote! {
                 #expr?;
-                dest.write_str(" ")?;
+                ::std::fmt::Write::write_str(dest, " ")?;
                 ::style_traits::ToCss::to_css(#binding, dest)
             };
         }
@@ -68,3 +71,40 @@ fn where_predicate(ty: syn::Ty) -> syn::WherePredicate {
         )],
     })
 }
+
+/// Transforms "FooBar" to "foo-bar".
+///
+/// If the first Camel segment is "Moz"" or "Webkit", the result string
+/// is prepended with "-".
+fn to_css_identifier(mut camel_case: &str) -> String {
+    let mut first = true;
+    let mut result = String::with_capacity(camel_case.len());
+    while let Some(segment) = split_camel_segment(&mut camel_case) {
+        if first {
+            match segment {
+                "Moz" | "Webkit" => first = false,
+                _ => {},
+            }
+        }
+        if !first {
+            result.push_str("-");
+        }
+        first = false;
+        result.push_str(&segment.to_lowercase());
+    }
+    result
+}
+
+/// Given "FooBar", returns "Foo" and sets `camel_case` to "Bar".
+fn split_camel_segment<'input>(camel_case: &mut &'input str) -> Option<&'input str> {
+    let index = match camel_case.chars().next() {
+        None => return None,
+        Some(ch) => ch.len_utf8(),
+    };
+    let end_position = camel_case[index..]
+        .find(char::is_uppercase)
+        .map_or(camel_case.len(), |pos| index + pos);
+    let result = &camel_case[..end_position];
+    *camel_case = &camel_case[end_position..];
+    Some(result)
+}
diff --git a/components/style_traits/values.rs b/components/style_traits/values.rs
index aaff1a998ea..7c73f7bd4cf 100644
--- a/components/style_traits/values.rs
+++ b/components/style_traits/values.rs
@@ -8,8 +8,15 @@ use app_units::Au;
 use cssparser::UnicodeRange;
 use std::fmt;
 
-/// The real `ToCss` trait can't be implemented for types in crates that don't
-/// depend on each other.
+/// Serialises a value according to its CSS representation.
+///
+/// This trait is derivable with `#[derive(ToCss)]`, with the following behaviour:
+/// * unit variants get serialised as the `snake-case` representation
+///   of their name;
+/// * unit variants whose name starts with "Moz" or "Webkit" are prepended
+///   with a "-";
+/// * variants with fields get serialised as the space-separated serialisations
+///   of their fields.
 pub trait ToCss {
     /// Serialize `self` in CSS syntax, writing to `dest`.
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write;