9 files changed, 49 insertions, 28 deletions
diff --git a/components/style/properties/gecko.mako.rs b/components/style/properties/gecko.mako.rs
index ca96e3fcb13..46e42442507 100644
--- a/components/style/properties/gecko.mako.rs
+++ b/components/style/properties/gecko.mako.rs
@@ -64,6 +64,7 @@ use values::computed::effects::{BoxShadow, Filter, SimpleShadow};
 use values::computed::outline::OutlineStyle;
 use values::generics::column::ColumnCount;
 use values::generics::position::ZIndex;
+use values::generics::text::MozTabSize;
 use values::generics::transform::TransformStyle;
 use computed_values::border_style;
 
@@ -4792,13 +4793,11 @@ fn static_assert() {
 
     #[allow(non_snake_case)]
     pub fn set__moz_tab_size(&mut self, v: longhands::_moz_tab_size::computed_value::T) {
-        use values::Either;
-
         match v {
-            Either::First(non_negative_number) => {
+            MozTabSize::Number(non_negative_number) => {
                 self.gecko.mTabSize.set_value(CoordDataValue::Factor(non_negative_number.0));
             }
-            Either::Second(non_negative_length) => {
+            MozTabSize::Length(non_negative_length) => {
                 self.gecko.mTabSize.set(non_negative_length);
             }
         }
@@ -4806,11 +4805,9 @@ fn static_assert() {
 
     #[allow(non_snake_case)]
     pub fn clone__moz_tab_size(&self) -> longhands::_moz_tab_size::computed_value::T {
-        use values::Either;
-
         match self.gecko.mTabSize.as_value() {
-            CoordDataValue::Coord(coord) => Either::Second(Au(coord).into()),
-            CoordDataValue::Factor(number) => Either::First(From::from(number)),
+            CoordDataValue::Coord(coord) => MozTabSize::Length(Au(coord).into()),
+            CoordDataValue::Factor(number) => MozTabSize::Number(From::from(number)),
             _ => unreachable!(),
         }
     }
diff --git a/components/style/properties/longhand/inherited_text.mako.rs b/components/style/properties/longhand/inherited_text.mako.rs
index 8bebe90c876..fbf656054ca 100644
--- a/components/style/properties/longhand/inherited_text.mako.rs
+++ b/components/style/properties/longhand/inherited_text.mako.rs
@@ -487,11 +487,13 @@ ${helpers.predefined_type(
 )}
 
 ${helpers.predefined_type(
-    "-moz-tab-size", "length::NonNegativeLengthOrNumber",
-    "::values::Either::First(From::from(8.0))",
-    products="gecko", animation_value_type="::values::computed::length::NonNegativeLengthOrNumber",
-    spec="https://drafts.csswg.org/css-text-3/#tab-size-property")}
-
+    "-moz-tab-size",
+    "MozTabSize",
+    "generics::text::MozTabSize::Number(From::from(8.0))",
+    products="gecko",
+    animation_value_type="AnimatedMozTabSize",
+    spec="https://drafts.csswg.org/css-text-3/#tab-size-property",
+)}
 
 // CSS Compatibility
 // https://compat.spec.whatwg.org
diff --git a/components/style/values/computed/length.rs b/components/style/values/computed/length.rs
index e52e01fbdc6..3b5e4d0c986 100644
--- a/components/style/values/computed/length.rs
+++ b/components/style/values/computed/length.rs
@@ -15,7 +15,6 @@ use style_traits::values::specified::AllowedNumericType;
 use super::{Number, ToComputedValue, Context, Percentage};
 use values::{Auto, CSSFloat, Either, Normal, specified};
 use values::animated::{Animate, Procedure, ToAnimatedValue, ToAnimatedZero};
-use values::computed::NonNegativeNumber;
 use values::distance::{ComputeSquaredDistance, SquaredDistance};
 use values::generics::NonNegative;
 use values::specified::length::{AbsoluteLength, FontBaseSize, FontRelativeLength};
@@ -898,9 +897,6 @@ pub type NonNegativeLengthOrAuto = Either<NonNegativeLength, Auto>;
 /// Either a computed NonNegativeLength or the `normal` keyword.
 pub type NonNegativeLengthOrNormal = Either<NonNegativeLength, Normal>;
 
-/// Either a computed NonNegativeLength or a NonNegativeNumber value.
-pub type NonNegativeLengthOrNumber = Either<NonNegativeNumber, NonNegativeLength>;
-
 /// A type for possible values for min- and max- flavors of width, height,
 /// block-size, and inline-size.
 #[allow(missing_docs)]
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index eb97d8ace15..6239e92e4a0 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -78,7 +78,8 @@ pub use self::svg::{SVGLength, SVGOpacity, SVGPaint, SVGPaintKind};
 pub use self::svg::{SVGPaintOrder, SVGStrokeDashArray, SVGWidth};
 pub use self::svg::MozContextProperties;
 pub use self::table::XSpan;
-pub use self::text::{InitialLetter, LetterSpacing, LineHeight, TextAlign, TextOverflow, WordSpacing};
+pub use self::text::{InitialLetter, LetterSpacing, LineHeight, MozTabSize};
+pub use self::text::{TextAlign, TextOverflow, WordSpacing};
 pub use self::time::Time;
 pub use self::transform::{Rotate, Scale, TimingFunction, Transform, TransformOperation};
 pub use self::transform::{TransformOrigin, TransformStyle, Translate};
diff --git a/components/style/values/computed/text.rs b/components/style/values/computed/text.rs
index 5de5e450cb6..011477ca781 100644
--- a/components/style/values/computed/text.rs
+++ b/components/style/values/computed/text.rs
@@ -13,6 +13,7 @@ use values::computed::{NonNegativeLength, NonNegativeNumber};
 use values::computed::length::{Length, LengthOrPercentage};
 use values::generics::text::InitialLetter as GenericInitialLetter;
 use values::generics::text::LineHeight as GenericLineHeight;
+use values::generics::text::MozTabSize as GenericMozTabSize;
 use values::generics::text::Spacing;
 use values::specified::text::{TextOverflowSide, TextDecorationLine};
 
@@ -147,3 +148,6 @@ impl TextDecorationsInEffect {
         result
     }
 }
+
+/// A specified value for the `-moz-tab-size` property.
+pub type MozTabSize = GenericMozTabSize<NonNegativeNumber, NonNegativeLength>;
diff --git a/components/style/values/generics/text.rs b/components/style/values/generics/text.rs
index 94955a58b02..fc121295c47 100644
--- a/components/style/values/generics/text.rs
+++ b/components/style/values/generics/text.rs
@@ -133,3 +133,13 @@ impl<N, L> LineHeight<N, L> {
         LineHeight::Normal
     }
 }
+
+/// A generic value for the `-moz-tab-size` property.
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug, MallocSizeOf)]
+#[derive(PartialEq, ToAnimatedValue, ToAnimatedZero, ToComputedValue, ToCss)]
+pub enum MozTabSize<Number, Length> {
+    /// A number.
+    Number(Number),
+    /// A length.
+    Length(Length),
+}
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 3ed3b4f5d30..1149e31c8ec 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -20,7 +20,6 @@ use super::{AllowQuirks, Number, ToComputedValue, Percentage};
 use values::{Auto, CSSFloat, Either, Normal};
 use values::computed::{self, CSSPixelLength, Context, ExtremumLength};
 use values::generics::NonNegative;
-use values::specified::NonNegativeNumber;
 use values::specified::calc::CalcNode;
 
 pub use values::specified::calc::CalcLengthOrPercentage;
@@ -687,12 +686,6 @@ pub type NonNegativeLengthOrNormal = Either<NonNegativeLength, Normal>;
 /// Either a NonNegativeLength or the `auto` keyword.
 pub type NonNegativeLengthOrAuto = Either<NonNegativeLength, Auto>;
 
-/// Either a NonNegativeLength or a NonNegativeNumber value.
-///
-/// The order is important, because `0` must be parsed as the number `0` and not
-/// the length `0px`.
-pub type NonNegativeLengthOrNumber = Either<NonNegativeNumber, NonNegativeLength>;
-
 /// A length or a percentage value.
 #[allow(missing_docs)]
 #[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
diff --git a/components/style/values/specified/mod.rs b/components/style/values/specified/mod.rs
index 7c22f840bda..35d141b5238 100644
--- a/components/style/values/specified/mod.rs
+++ b/components/style/values/specified/mod.rs
@@ -72,8 +72,8 @@ pub use self::svg::{SVGLength, SVGOpacity, SVGPaint, SVGPaintKind};
 pub use self::svg::{SVGPaintOrder, SVGStrokeDashArray, SVGWidth};
 pub use self::svg::MozContextProperties;
 pub use self::table::XSpan;
-pub use self::text::{InitialLetter, LetterSpacing, LineHeight, TextDecorationLine};
-pub use self::text::{TextAlign, TextAlignKeyword, TextOverflow, WordSpacing};
+pub use self::text::{InitialLetter, LetterSpacing, LineHeight, MozTabSize, TextAlign};
+pub use self::text::{TextAlignKeyword, TextDecorationLine, TextOverflow, WordSpacing};
 pub use self::time::Time;
 pub use self::transform::{Rotate, Scale, TimingFunction, Transform};
 pub use self::transform::{TransformOrigin, TransformStyle, Translate};
diff --git a/components/style/values/specified/text.rs b/components/style/values/specified/text.rs
index 41ebe0cce86..847cd5848cf 100644
--- a/components/style/values/specified/text.rs
+++ b/components/style/values/specified/text.rs
@@ -15,10 +15,11 @@ use values::computed::text::LineHeight as ComputedLineHeight;
 use values::computed::text::TextOverflow as ComputedTextOverflow;
 use values::generics::text::InitialLetter as GenericInitialLetter;
 use values::generics::text::LineHeight as GenericLineHeight;
+use values::generics::text::MozTabSize as GenericMozTabSize;
 use values::generics::text::Spacing;
 use values::specified::{AllowQuirks, Integer, NonNegativeNumber, Number};
 use values::specified::length::{FontRelativeLength, Length, LengthOrPercentage, NoCalcLength};
-use values::specified::length::NonNegativeLengthOrPercentage;
+use values::specified::length::{NonNegativeLength, NonNegativeLengthOrPercentage};
 
 /// A specified type for the `initial-letter` property.
 pub type InitialLetter = GenericInitialLetter<Number, Integer>;
@@ -514,3 +515,20 @@ impl ToComputedValue for TextAlign {
         TextAlign::Keyword(*computed)
     }
 }
+
+/// A specified value for the `-moz-tab-size` property.
+pub type MozTabSize = GenericMozTabSize<NonNegativeNumber, NonNegativeLength>;
+
+impl Parse for MozTabSize {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        if let Ok(number) = input.try(|i| NonNegativeNumber::parse(context, i)) {
+            // Numbers need to be parsed first because `0` must be recognised
+            // as the number `0` and not the length `0px`.
+            return Ok(GenericMozTabSize::Number(number));
+        }
+        Ok(GenericMozTabSize::Length(NonNegativeLength::parse(context, input)?))
+    }
+}