diff --git a/components/style/properties/gecko.mako.rs b/components/style/properties/gecko.mako.rs
index b906cfa808c..481ea4e9222 100644
--- a/components/style/properties/gecko.mako.rs
+++ b/components/style/properties/gecko.mako.rs
@@ -4795,10 +4795,10 @@ fn static_assert() {
         use values::Either;
 
         match v {
-            Either::Second(non_negative_number) => {
+            Either::First(non_negative_number) => {
                 self.gecko.mTabSize.set_value(CoordDataValue::Factor(non_negative_number.0));
             }
-            Either::First(non_negative_length) => {
+            Either::Second(non_negative_length) => {
                 self.gecko.mTabSize.set(non_negative_length);
             }
         }
@@ -4809,8 +4809,8 @@ fn static_assert() {
         use values::Either;
 
         match self.gecko.mTabSize.as_value() {
-            CoordDataValue::Coord(coord) => Either::First(Au(coord).into()),
-            CoordDataValue::Factor(number) => Either::Second(From::from(number)),
+            CoordDataValue::Coord(coord) => Either::Second(Au(coord).into()),
+            CoordDataValue::Factor(number) => Either::First(From::from(number)),
             _ => unreachable!(),
         }
     }
diff --git a/components/style/properties/longhand/box.mako.rs b/components/style/properties/longhand/box.mako.rs
index 6bc9bf8e621..daf2abd0abe 100644
--- a/components/style/properties/longhand/box.mako.rs
+++ b/components/style/properties/longhand/box.mako.rs
@@ -511,7 +511,6 @@ ${helpers.single_keyword("resize",
 ${helpers.predefined_type("perspective",
                           "LengthOrNone",
                           "Either::Second(None_)",
-                          "parse_non_negative_length",
                           gecko_ffi_name="mChildPerspective",
                           spec="https://drafts.csswg.org/css-transforms/#perspective",
                           extra_prefixes="moz webkit",
diff --git a/components/style/properties/longhand/inherited_text.mako.rs b/components/style/properties/longhand/inherited_text.mako.rs
index 3594cab8463..8bebe90c876 100644
--- a/components/style/properties/longhand/inherited_text.mako.rs
+++ b/components/style/properties/longhand/inherited_text.mako.rs
@@ -488,7 +488,7 @@ ${helpers.predefined_type(
 
 ${helpers.predefined_type(
     "-moz-tab-size", "length::NonNegativeLengthOrNumber",
-    "::values::Either::Second(From::from(8.0))",
+    "::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")}
 
diff --git a/components/style/values/computed/length.rs b/components/style/values/computed/length.rs
index b482c8aeb17..002e863f757 100644
--- a/components/style/values/computed/length.rs
+++ b/components/style/values/computed/length.rs
@@ -902,7 +902,7 @@ pub type NonNegativeLengthOrAuto = Either<NonNegativeLength, Auto>;
 pub type NonNegativeLengthOrNormal = Either<NonNegativeLength, Normal>;
 
 /// Either a computed NonNegativeLength or a NonNegativeNumber value.
-pub type NonNegativeLengthOrNumber = Either<NonNegativeLength, NonNegativeNumber>;
+pub type NonNegativeLengthOrNumber = Either<NonNegativeNumber, NonNegativeLength>;
 
 /// A type for possible values for min- and max- flavors of width, height,
 /// block-size, and inline-size.
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 0db823acd45..aa2ac34e61e 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -640,21 +640,19 @@ impl Length {
     }
 }
 
-impl<T: Parse> Either<Length, T> {
-    /// Parse a non-negative length
-    #[inline]
-    pub fn parse_non_negative_length<'i, 't>(context: &ParserContext, input: &mut Parser<'i, 't>)
-                                             -> Result<Self, ParseError<'i>> {
-        if let Ok(v) = input.try(|input| T::parse(context, input)) {
-            return Ok(Either::Second(v));
-        }
-        Length::parse_internal(context, input, AllowedNumericType::NonNegative, AllowQuirks::No).map(Either::First)
-    }
-}
-
 /// A wrapper of Length, whose value must be >= 0.
 pub type NonNegativeLength = NonNegative<Length>;
 
+impl Parse for NonNegativeLength {
+    #[inline]
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        Ok(NonNegative(Length::parse_non_negative(context, input)?))
+    }
+}
+
 impl From<NoCalcLength> for NonNegativeLength {
     #[inline]
     fn from(len: NoCalcLength) -> Self {
@@ -669,17 +667,6 @@ impl From<Length> for NonNegativeLength {
     }
 }
 
-impl<T: Parse> Parse for Either<NonNegativeLength, T> {
-    #[inline]
-    fn parse<'i, 't>(context: &ParserContext, input: &mut Parser<'i, 't>) -> Result<Self, ParseError<'i>> {
-        if let Ok(v) = input.try(|input| T::parse(context, input)) {
-            return Ok(Either::Second(v));
-        }
-        Length::parse_internal(context, input, AllowedNumericType::NonNegative, AllowQuirks::No)
-            .map(NonNegative::<Length>).map(Either::First)
-    }
-}
-
 impl NonNegativeLength {
     /// Returns a `zero` length.
     #[inline]
@@ -701,7 +688,10 @@ pub type NonNegativeLengthOrNormal = Either<NonNegativeLength, Normal>;
 pub type NonNegativeLengthOrAuto = Either<NonNegativeLength, Auto>;
 
 /// Either a NonNegativeLength or a NonNegativeNumber value.
-pub type NonNegativeLengthOrNumber = Either<NonNegativeLength, NonNegativeNumber>;
+///
+/// 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)]