diff --git a/components/style/properties/longhand/background.mako.rs b/components/style/properties/longhand/background.mako.rs
index 66db18d6447..b053be48fbd 100644
--- a/components/style/properties/longhand/background.mako.rs
+++ b/components/style/properties/longhand/background.mako.rs
@@ -483,36 +483,23 @@ ${helpers.single_keyword("background-origin",
     }
 
     pub fn parse(_context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue,()> {
-        let width;
-        if let Ok(value) = input.try(|input| {
-            match input.next() {
-                Err(_) => Err(()),
-                Ok(Token::Ident(ref ident)) if ident.eq_ignore_ascii_case("cover") => {
-                    Ok(SpecifiedValue::Cover)
-                }
-                Ok(Token::Ident(ref ident)) if ident.eq_ignore_ascii_case("contain") => {
-                    Ok(SpecifiedValue::Contain)
-                }
-                Ok(_) => Err(()),
-            }
-        }) {
-            return Ok(value)
-        } else {
-            width = try!(specified::LengthOrPercentageOrAuto::parse_non_negative(input))
+        if input.try(|input| input.expect_ident_matching("cover")).is_ok() {
+            return Ok(SpecifiedValue::Cover);
         }
 
-        let height;
-        if let Ok(value) = input.try(|input| {
-            match input.next() {
-                Err(_) => Ok(specified::LengthOrPercentageOrAuto::Auto),
-                Ok(_) => Err(()),
-            }
-        }) {
-            height = value
-        } else {
-            height = try!(specified::LengthOrPercentageOrAuto::parse_non_negative(input));
+        if input.try(|input| input.expect_ident_matching("contain")).is_ok() {
+            return Ok(SpecifiedValue::Contain);
         }
 
+        let width =
+            try!(specified::LengthOrPercentageOrAuto::parse_non_negative(input));
+
+        let height = if input.is_exhausted() {
+            specified::LengthOrPercentageOrAuto::Auto
+        } else {
+            try!(specified::LengthOrPercentageOrAuto::parse_non_negative(input))
+        };
+
         Ok(SpecifiedValue::Explicit(ExplicitSize {
             width: width,
             height: height,
diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index abceff2dcd5..a300fae7900 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -659,15 +659,17 @@ ${helpers.single_keyword("font-variant-caps",
     /// <length> | <percentage> | <absolute-size> | <relative-size>
     pub fn parse(_: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
         if let Ok(lop) = input.try(specified::LengthOrPercentage::parse_non_negative) {
-            Ok(SpecifiedValue::Length(lop))
-        } else if let Ok(kw) = input.try(KeywordSize::parse) {
-            Ok(SpecifiedValue::Keyword(kw))
-        } else {
-            match_ignore_ascii_case! {&*input.expect_ident()?,
-                "smaller" => Ok(SpecifiedValue::Smaller),
-                "larger" => Ok(SpecifiedValue::Larger),
-                _ => Err(())
-            }
+            return Ok(SpecifiedValue::Length(lop))
+        }
+
+        if let Ok(kw) = input.try(KeywordSize::parse) {
+            return Ok(SpecifiedValue::Keyword(kw))
+        }
+
+        match_ignore_ascii_case! {&*input.expect_ident()?,
+            "smaller" => Ok(SpecifiedValue::Smaller),
+            "larger" => Ok(SpecifiedValue::Larger),
+            _ => Err(())
         }
     }
 </%helpers:longhand>
diff --git a/components/style/properties/longhand/inherited_text.mako.rs b/components/style/properties/longhand/inherited_text.mako.rs
index baf8e0ecb8d..47075cbc8cd 100644
--- a/components/style/properties/longhand/inherited_text.mako.rs
+++ b/components/style/properties/longhand/inherited_text.mako.rs
@@ -45,31 +45,33 @@
         }
     }
     /// normal | <number> | <length> | <percentage>
-    pub fn parse(_context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
+    pub fn parse(_: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
         use cssparser::Token;
         use std::ascii::AsciiExt;
-        // We try to parse as a Number first because, for 'line-height', we want "0" to be
-        // parsed as a plain Number rather than a Length (0px); this matches the behaviour
-        // of all major browsers
-        input.try(specified::Number::parse_non_negative)
-            .map(SpecifiedValue::Number)
-            .or_else(|()| {
-                input.try(specified::LengthOrPercentage::parse_non_negative)
-                .map(SpecifiedValue::LengthOrPercentage)
-                .or_else(|()| {
-                    match try!(input.next()) {
-                        Token::Ident(ref value) if value.eq_ignore_ascii_case("normal") => {
-                            Ok(SpecifiedValue::Normal)
-                        }
-                        % if product == "gecko":
-                        Token::Ident(ref value) if value.eq_ignore_ascii_case("-moz-block-height") => {
-                            Ok(SpecifiedValue::MozBlockHeight)
-                        }
-                        % endif
-                        _ => Err(()),
-                    }
-                })
-            })
+
+        // We try to parse as a Number first because, for 'line-height', we want
+        // "0" to be parsed as a plain Number rather than a Length (0px); this
+        // matches the behaviour of all major browsers
+        if let Ok(number) = input.try(specified::Number::parse_non_negative) {
+            return Ok(SpecifiedValue::Number(number))
+        }
+
+        if let Ok(lop) = input.try(specified::LengthOrPercentage::parse_non_negative) {
+            return Ok(SpecifiedValue::LengthOrPercentage(lop))
+        }
+
+
+        match try!(input.next()) {
+            Token::Ident(ref value) if value.eq_ignore_ascii_case("normal") => {
+                Ok(SpecifiedValue::Normal)
+            }
+            % if product == "gecko":
+            Token::Ident(ref value) if value.eq_ignore_ascii_case("-moz-block-height") => {
+                Ok(SpecifiedValue::MozBlockHeight)
+            }
+            % endif
+            _ => Err(()),
+        }
     }
     pub mod computed_value {
         use app_units::Au;
diff --git a/components/style/properties/longhand/position.mako.rs b/components/style/properties/longhand/position.mako.rs
index c4a03e15deb..a5df58b28af 100644
--- a/components/style/properties/longhand/position.mako.rs
+++ b/components/style/properties/longhand/position.mako.rs
@@ -142,7 +142,8 @@ ${helpers.predefined_type("flex-basis",
                           "LengthOrPercentageOrAutoOrContent",
                           "computed::LengthOrPercentageOrAuto::Auto" if product == "gecko" else
                           "computed::LengthOrPercentageOrAutoOrContent::Auto",
-                          "parse_non_negative_with_context",
+                          "parse_non_negative",
+                          needs_context=False,
                           spec="https://drafts.csswg.org/css-flexbox/#flex-basis-property",
                           extra_prefixes="webkit",
                           animatable=True if product == "gecko" else False)}
diff --git a/components/style/properties/shorthand/position.mako.rs b/components/style/properties/shorthand/position.mako.rs
index 825a63d3478..3d4009d750f 100644
--- a/components/style/properties/shorthand/position.mako.rs
+++ b/components/style/properties/shorthand/position.mako.rs
@@ -48,16 +48,10 @@
 
 <%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 values::specified::{Number, NoCalcLength};
-    % if product == "gecko":
-        use values::specified::LengthOrPercentageOrAuto;
-    % else:
-        use values::specified::LengthOrPercentageOrAutoOrContent;
-    % endif
+    use values::specified::Number;
 
-    pub fn parse_flexibility(input: &mut Parser)
-                             -> Result<(Number, Option<Number>),()> {
+    fn parse_flexibility(input: &mut Parser)
+                         -> Result<(Number, Option<Number>),()> {
         let grow = try!(Number::parse_non_negative(input));
         let shrink = input.try(Number::parse_non_negative).ok();
         Ok((grow, shrink))
@@ -72,12 +66,7 @@
             return Ok(Longhands {
                 flex_grow: Number::new(0.0),
                 flex_shrink: Number::new(0.0),
-                % if product == "gecko":
-                    flex_basis: LengthOrPercentageOrAuto::Auto
-                % else:
-                    flex_basis: LengthOrPercentageOrAutoOrContent::Auto
-                % endif
-
+                flex_basis: longhands::flex_basis::SpecifiedValue::auto(),
             })
         }
         loop {
@@ -89,11 +78,7 @@
                 }
             }
             if basis.is_none() {
-                % if product == "gecko":
-                    if let Ok(value) = input.try(|i| LengthOrPercentageOrAuto::parse(context, i)) {
-                % else:
-                    if let Ok(value) = input.try(|i| LengthOrPercentageOrAutoOrContent::parse(context, i)) {
-                % endif
+                if let Ok(value) = input.try(|input| longhands::flex_basis::parse(context, input)) {
                     basis = Some(value);
                     continue
                 }
@@ -107,21 +92,17 @@
         Ok(Longhands {
             flex_grow: grow.unwrap_or(Number::new(1.0)),
             flex_shrink: shrink.unwrap_or(Number::new(1.0)),
-            % if product == "gecko":
-                flex_basis: basis.unwrap_or(LengthOrPercentageOrAuto::Length(NoCalcLength::zero()))
-            % else:
-                flex_basis: basis.unwrap_or(LengthOrPercentageOrAutoOrContent::Length(NoCalcLength::zero()))
-            % endif
+            flex_basis: basis.unwrap_or(longhands::flex_basis::SpecifiedValue::zero()),
         })
     }
 
     impl<'a> ToCss for LonghandsToSerialize<'a>  {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
             try!(self.flex_grow.to_css(dest));
-            try!(write!(dest, " "));
+            try!(dest.write_str(" "));
 
             try!(self.flex_shrink.to_css(dest));
-            try!(write!(dest, " "));
+            try!(dest.write_str(" "));
 
             self.flex_basis.to_css(dest)
         }
diff --git a/components/style/stylesheets.rs b/components/style/stylesheets.rs
index f59aceb9e61..865b2fb4b75 100644
--- a/components/style/stylesheets.rs
+++ b/components/style/stylesheets.rs
@@ -23,7 +23,7 @@ use gecko_bindings::sugar::refptr::RefPtr;
 use keyframes::{Keyframe, parse_keyframe_list};
 use media_queries::{Device, MediaList, parse_media_query_list};
 use parking_lot::RwLock;
-use parser::{ParserContext, log_css_error};
+use parser::{Parse, ParserContext, log_css_error};
 use properties::{PropertyDeclarationBlock, parse_property_declaration_list};
 use selector_parser::{SelectorImpl, SelectorParser};
 use selectors::parser::SelectorList;
@@ -1059,7 +1059,7 @@ impl<'a, 'b> AtRuleParser for NestedRuleParser<'a, 'b> {
             }
             AtRulePrelude::Viewport => {
                 Ok(CssRule::Viewport(Arc::new(self.shared_lock.wrap(
-                    try!(ViewportRule::parse(input, self.context))))))
+                    try!(ViewportRule::parse(self.context, input))))))
             }
             AtRulePrelude::Keyframes(name) => {
                 Ok(CssRule::Keyframes(Arc::new(self.shared_lock.wrap(KeyframesRule {
diff --git a/components/style/values/specified/grid.rs b/components/style/values/specified/grid.rs
index 5ceb16dd889..e04e85b9394 100644
--- a/components/style/values/specified/grid.rs
+++ b/components/style/values/specified/grid.rs
@@ -6,6 +6,7 @@
 
 use cssparser::{Parser, Token};
 use parser::{Parse, ParserContext};
+use std::ascii::AsciiExt;
 use std::fmt;
 use style_traits::ToCss;
 use values::{CSSFloat, HasViewportPercentage};
@@ -130,23 +131,23 @@ pub enum TrackBreadth<L> {
 /// Parse a single flexible length.
 pub fn parse_flex(input: &mut Parser) -> Result<CSSFloat, ()> {
     match try!(input.next()) {
-        Token::Dimension(ref value, ref unit) if unit.to_lowercase() == "fr" && value.value.is_sign_positive()
+        Token::Dimension(ref value, ref unit) if unit.eq_ignore_ascii_case("fr") && value.value.is_sign_positive()
             => Ok(value.value),
         _ => Err(()),
     }
 }
 
 impl Parse for TrackBreadth<LengthOrPercentage> {
-    fn parse(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+    fn parse(_: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
         if let Ok(lop) = input.try(LengthOrPercentage::parse_non_negative) {
-            Ok(TrackBreadth::Breadth(lop))
-        } else {
-            if let Ok(f) = input.try(parse_flex) {
-                Ok(TrackBreadth::Flex(f))
-            } else {
-                TrackKeyword::parse(input).map(TrackBreadth::Keyword)
-            }
+            return Ok(TrackBreadth::Breadth(lop))
         }
+
+        if let Ok(f) = input.try(parse_flex) {
+            return Ok(TrackBreadth::Flex(f))
+        }
+
+        TrackKeyword::parse(input).map(TrackBreadth::Keyword)
     }
 }
 
@@ -223,24 +224,28 @@ impl<L> Default for TrackSize<L> {
 impl Parse for TrackSize<LengthOrPercentage> {
     fn parse(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
         if let Ok(b) = input.try(|i| TrackBreadth::parse(context, i)) {
-            Ok(TrackSize::Breadth(b))
-        } else {
-            if input.try(|i| i.expect_function_matching("minmax")).is_ok() {
-                Ok(try!(input.parse_nested_block(|input| {
-                    let inflexible_breadth = if let Ok(lop) = input.try(LengthOrPercentage::parse_non_negative) {
-                        Ok(TrackBreadth::Breadth(lop))
-                    } else {
-                        TrackKeyword::parse(input).map(TrackBreadth::Keyword)
+            return Ok(TrackSize::Breadth(b))
+        }
+
+        if input.try(|i| i.expect_function_matching("minmax")).is_ok() {
+            return input.parse_nested_block(|input| {
+                let inflexible_breadth =
+                    match input.try(LengthOrPercentage::parse_non_negative) {
+                        Ok(lop) => TrackBreadth::Breadth(lop),
+                        Err(..) => {
+                            let keyword = try!(TrackKeyword::parse(input));
+                            TrackBreadth::Keyword(keyword)
+                        }
                     };
 
-                    try!(input.expect_comma());
-                    Ok(TrackSize::MinMax(try!(inflexible_breadth), try!(TrackBreadth::parse(context, input))))
-                })))
-            } else {
-                try!(input.expect_function_matching("fit-content"));
-                Ok(try!(LengthOrPercentage::parse(context, input).map(TrackSize::FitContent)))
-            }
+                try!(input.expect_comma());
+                Ok(TrackSize::MinMax(inflexible_breadth, try!(TrackBreadth::parse(context, input))))
+            });
         }
+
+        try!(input.expect_function_matching("fit-content"));
+        // FIXME(emilio): This needs a parse_nested_block, doesn't it?
+        Ok(try!(LengthOrPercentage::parse(context, input).map(TrackSize::FitContent)))
     }
 }
 
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index ed31cc572e8..5e96e0e40f6 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -442,7 +442,7 @@ impl Length {
     /// Parse a non-negative length
     #[inline]
     pub fn parse_non_negative(input: &mut Parser) -> Result<Length, ()> {
-        Length::parse_internal(input, AllowedNumericType::NonNegative)
+        Self::parse_internal(input, AllowedNumericType::NonNegative)
     }
 
     /// Get an absolute length from a px value.
@@ -462,7 +462,7 @@ impl Length {
 
 impl Parse for Length {
     fn parse(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
-        Length::parse_internal(input, AllowedNumericType::All)
+        Self::parse_internal(input, AllowedNumericType::All)
     }
 }
 
@@ -1049,7 +1049,7 @@ impl LengthOrPercentage {
     /// Parse a non-negative length.
     #[inline]
     pub fn parse_non_negative(input: &mut Parser) -> Result<LengthOrPercentage, ()> {
-        LengthOrPercentage::parse_internal(input, AllowedNumericType::NonNegative)
+        Self::parse_internal(input, AllowedNumericType::NonNegative)
     }
 
     /// Parse a length, treating dimensionless numbers as pixels
@@ -1057,11 +1057,13 @@ impl LengthOrPercentage {
     /// https://www.w3.org/TR/SVG2/types.html#presentation-attribute-css-value
     pub fn parse_numbers_are_pixels(input: &mut Parser) -> Result<LengthOrPercentage, ()> {
         if let Ok(lop) = input.try(|i| Self::parse_internal(i, AllowedNumericType::All)) {
-            Ok(lop)
-        } else {
-            let num = input.expect_number()?;
-            Ok(LengthOrPercentage::Length(NoCalcLength::Absolute(Au((AU_PER_PX * num) as i32))))
+            return Ok(lop)
         }
+
+        // TODO(emilio): Probably should use Number::parse_non_negative to
+        // handle calc()?
+        let num = input.expect_number()?;
+        Ok(LengthOrPercentage::Length(NoCalcLength::Absolute(Au((AU_PER_PX * num) as i32))))
     }
 
     /// Parse a non-negative length, treating dimensionless numbers as pixels
@@ -1069,14 +1071,16 @@ impl LengthOrPercentage {
     /// This is nonstandard behavior used by Firefox for SVG
     pub fn parse_numbers_are_pixels_non_negative(input: &mut Parser) -> Result<LengthOrPercentage, ()> {
         if let Ok(lop) = input.try(|i| Self::parse_internal(i, AllowedNumericType::NonNegative)) {
-            Ok(lop)
+            return Ok(lop)
+        }
+
+        // TODO(emilio): Probably should use Number::parse_non_negative to
+        // handle calc()?
+        let num = input.expect_number()?;
+        if num >= 0. {
+            Ok(LengthOrPercentage::Length(NoCalcLength::Absolute(Au((AU_PER_PX * num) as i32))))
         } else {
-            let num = input.expect_number()?;
-            if num >= 0. {
-                Ok(LengthOrPercentage::Length(NoCalcLength::Absolute(Au((AU_PER_PX * num) as i32))))
-            } else {
-                Err(())
-            }
+            Err(())
         }
     }
 
@@ -1092,7 +1096,7 @@ impl LengthOrPercentage {
 impl Parse for LengthOrPercentage {
     #[inline]
     fn parse(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
-        LengthOrPercentage::parse_internal(input, AllowedNumericType::All)
+        Self::parse_internal(input, AllowedNumericType::All)
     }
 }
 
@@ -1146,15 +1150,14 @@ impl ToCss for LengthOrPercentageOrAuto {
 
 impl LengthOrPercentageOrAuto {
     fn parse_internal(input: &mut Parser, context: AllowedNumericType)
-                      -> Result<LengthOrPercentageOrAuto, ()>
-    {
+                      -> Result<Self, ()> {
         match try!(input.next()) {
             Token::Dimension(ref value, ref unit) if context.is_ok(value.value) =>
                 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(NoCalcLength::zero())),
+                Ok(Self::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") => {
@@ -1168,22 +1171,24 @@ impl LengthOrPercentageOrAuto {
     /// Parse a non-negative length, percentage, or auto.
     #[inline]
     pub fn parse_non_negative(input: &mut Parser) -> Result<LengthOrPercentageOrAuto, ()> {
-        LengthOrPercentageOrAuto::parse_internal(input, AllowedNumericType::NonNegative)
+        Self::parse_internal(input, AllowedNumericType::NonNegative)
     }
 
-    /// Parse a non-negative length, percentage, or auto.
-    #[inline]
-    pub fn parse_non_negative_with_context(_context: &ParserContext,
-                                           input: &mut Parser)
-                                           -> Result<LengthOrPercentageOrAuto, ()> {
-        LengthOrPercentageOrAuto::parse_non_negative(input)
+    /// Returns the `auto` value.
+    pub fn auto() -> Self {
+        LengthOrPercentageOrAuto::Auto
+    }
+
+    /// Returns a value representing a `0` length.
+    pub fn zero() -> Self {
+        LengthOrPercentageOrAuto::Length(NoCalcLength::zero())
     }
 }
 
 impl Parse for LengthOrPercentageOrAuto {
     #[inline]
     fn parse(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
-        LengthOrPercentageOrAuto::parse_internal(input, AllowedNumericType::All)
+        Self::parse_internal(input, AllowedNumericType::All)
     }
 }
 
@@ -1241,15 +1246,15 @@ impl LengthOrPercentageOrNone {
     }
     /// Parse a non-negative LengthOrPercentageOrNone.
     #[inline]
-    pub fn parse_non_negative(input: &mut Parser) -> Result<LengthOrPercentageOrNone, ()> {
-        LengthOrPercentageOrNone::parse_internal(input, AllowedNumericType::NonNegative)
+    pub fn parse_non_negative(input: &mut Parser) -> Result<Self, ()> {
+        Self::parse_internal(input, AllowedNumericType::NonNegative)
     }
 }
 
 impl Parse for LengthOrPercentageOrNone {
     #[inline]
     fn parse(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
-        LengthOrPercentageOrNone::parse_internal(input, AllowedNumericType::All)
+        Self::parse_internal(input, AllowedNumericType::All)
     }
 }
 
@@ -1282,12 +1287,36 @@ pub enum LengthOrPercentageOrAutoOrContent {
 }
 
 impl LengthOrPercentageOrAutoOrContent {
-    /// Alias to `parse` so that Gecko and Servo can use the same method name for
-    /// both `LengthOrPercentageOrAuto` and `LengthOrPercentageOrAutoOrContent`.
-    ///
-    /// NOTE: `parse` already only accepts non-negative values.
-    pub fn parse_non_negative_with_context(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
-        Self::parse(context, input)
+    /// Parse a non-negative LengthOrPercentageOrAutoOrContent.
+    pub fn parse_non_negative(input: &mut Parser) -> Result<Self, ()> {
+        let context = AllowedNumericType::NonNegative;
+        match try!(input.next()) {
+            Token::Dimension(ref value, ref unit) if context.is_ok(value.value) =>
+                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(Self::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") =>
+                Ok(LengthOrPercentageOrAutoOrContent::Content),
+            Token::Function(ref name) if name.eq_ignore_ascii_case("calc") => {
+                let calc = try!(input.parse_nested_block(CalcLengthOrPercentage::parse_length_or_percentage));
+                Ok(LengthOrPercentageOrAutoOrContent::Calc(Box::new(calc)))
+            },
+            _ => Err(())
+        }
+    }
+
+    /// Returns the `auto` value.
+    pub fn auto() -> Self {
+        LengthOrPercentageOrAutoOrContent::Auto
+    }
+
+    /// Returns a value representing a `0` length.
+    pub fn zero() -> Self {
+        LengthOrPercentageOrAutoOrContent::Length(NoCalcLength::zero())
     }
 }
 
@@ -1313,43 +1342,20 @@ impl ToCss for LengthOrPercentageOrAutoOrContent {
     }
 }
 
-impl Parse for LengthOrPercentageOrAutoOrContent {
-    fn parse(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
-        let context = AllowedNumericType::NonNegative;
-        match try!(input.next()) {
-            Token::Dimension(ref value, ref unit) if context.is_ok(value.value) =>
-                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(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") =>
-                Ok(LengthOrPercentageOrAutoOrContent::Content),
-            Token::Function(ref name) if name.eq_ignore_ascii_case("calc") => {
-                let calc = try!(input.parse_nested_block(CalcLengthOrPercentage::parse_length_or_percentage));
-                Ok(LengthOrPercentageOrAutoOrContent::Calc(Box::new(calc)))
-            },
-            _ => Err(())
-        }
-    }
-}
-
 /// Either a `<length>` or a `<number>`.
 pub type LengthOrNumber = Either<Length, Number>;
 
 impl LengthOrNumber {
     /// Parse a non-negative LengthOrNumber.
-    pub fn parse_non_negative(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
-        // We try to parse as a Number first because, for cases like LengthOrNumber,
-        // we want "0" to be parsed as a plain Number rather than a Length (0px); this
-        // matches the behaviour of all major browsers
+    pub fn parse_non_negative(_: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+        // We try to parse as a Number first because, for cases like
+        // LengthOrNumber, we want "0" to be parsed as a plain Number rather
+        // than a Length (0px); this matches the behaviour of all major browsers
         if let Ok(v) = input.try(Number::parse_non_negative) {
-            Ok(Either::Second(v))
-        } else {
-            Length::parse_non_negative(input).map(Either::First)
+            return Ok(Either::Second(v))
         }
+
+        Length::parse_non_negative(input).map(Either::First)
     }
 }
 
diff --git a/components/style/viewport.rs b/components/style/viewport.rs
index 690c06fd377..d2d2af77480 100644
--- a/components/style/viewport.rs
+++ b/components/style/viewport.rs
@@ -14,7 +14,7 @@ use cssparser::{AtRuleParser, DeclarationListParser, DeclarationParser, Parser,
 use cssparser::ToCss as ParserToCss;
 use euclid::size::TypedSize2D;
 use media_queries::Device;
-use parser::{ParserContext, log_css_error};
+use parser::{Parse, ParserContext, log_css_error};
 use shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
 use std::ascii::AsciiExt;
 use std::borrow::Cow;
@@ -165,9 +165,9 @@ impl FromMeta for ViewportLength {
 }
 
 impl ViewportLength {
-    fn parse(input: &mut Parser) -> Result<ViewportLength, ()> {
-        // we explicitly do not accept 'extend-to-zoom', since it is a UA internal value
-        // for <META> viewport translation
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        // we explicitly do not accept 'extend-to-zoom', since it is a UA
+        // internal value for <META> viewport translation
         LengthOrPercentageOrAuto::parse_non_negative(input).map(ViewportLength::Specified)
     }
 }
@@ -246,7 +246,7 @@ impl ToCss for ViewportDescriptorDeclaration {
 
 fn parse_shorthand(input: &mut Parser) -> Result<(ViewportLength, ViewportLength), ()> {
     let min = try!(ViewportLength::parse(input));
-    match input.try(|input| ViewportLength::parse(input)) {
+    match input.try(ViewportLength::parse) {
         Err(()) => Ok((min.clone(), min)),
         Ok(max) => Ok((min, max))
     }
@@ -287,33 +287,18 @@ impl<'a, 'b> DeclarationParser for ViewportRuleParser<'a, 'b> {
             }}
         }
 
-        match name {
-            n if n.eq_ignore_ascii_case("min-width") =>
-                ok!(MinWidth(ViewportLength::parse)),
-            n if n.eq_ignore_ascii_case("max-width") =>
-                ok!(MaxWidth(ViewportLength::parse)),
-            n if n.eq_ignore_ascii_case("width") =>
-                ok!(shorthand -> [MinWidth, MaxWidth]),
-
-            n if n.eq_ignore_ascii_case("min-height") =>
-                ok!(MinHeight(ViewportLength::parse)),
-            n if n.eq_ignore_ascii_case("max-height") =>
-                ok!(MaxHeight(ViewportLength::parse)),
-            n if n.eq_ignore_ascii_case("height") =>
-                ok!(shorthand -> [MinHeight, MaxHeight]),
-
-            n if n.eq_ignore_ascii_case("zoom") =>
-                ok!(Zoom(Zoom::parse)),
-            n if n.eq_ignore_ascii_case("min-zoom") =>
-                ok!(MinZoom(Zoom::parse)),
-            n if n.eq_ignore_ascii_case("max-zoom") =>
-                ok!(MaxZoom(Zoom::parse)),
-
-            n if n.eq_ignore_ascii_case("user-zoom") =>
-                ok!(UserZoom(UserZoom::parse)),
-            n if n.eq_ignore_ascii_case("orientation") =>
-                ok!(Orientation(Orientation::parse)),
-
+        match_ignore_ascii_case! { name,
+            "min-width" => ok!(MinWidth(ViewportLength::parse)),
+            "max-width" => ok!(MaxWidth(ViewportLength::parse)),
+            "width" => ok!(shorthand -> [MinWidth, MaxWidth]),
+            "min-height" => ok!(MinHeight(ViewportLength::parse)),
+            "max-height" => ok!(MaxHeight(ViewportLength::parse)),
+            "height" => ok!(shorthand -> [MinHeight, MaxHeight]),
+            "zoom" => ok!(Zoom(Zoom::parse)),
+            "min-zoom" => ok!(MinZoom(Zoom::parse)),
+            "max-zoom" => ok!(MaxZoom(Zoom::parse)),
+            "user-zoom" => ok!(UserZoom(UserZoom::parse)),
+            "orientation" => ok!(Orientation(Orientation::parse)),
             _ => Err(()),
         }
     }
@@ -340,11 +325,9 @@ fn is_whitespace_separator_or_equals(c: &char) -> bool {
     WHITESPACE.contains(c) || SEPARATOR.contains(c) || *c == '='
 }
 
-impl ViewportRule {
+impl Parse for ViewportRule {
     #[allow(missing_docs)]
-    pub fn parse(input: &mut Parser, context: &ParserContext)
-                     -> Result<ViewportRule, ()>
-    {
+    fn parse(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
         let parser = ViewportRuleParser { context: context };
 
         let mut cascade = Cascade::new();
@@ -366,7 +349,9 @@ impl ViewportRule {
         }
         Ok(ViewportRule { declarations: cascade.finish() })
     }
+}
 
+impl ViewportRule {
     #[allow(missing_docs)]
     pub fn from_meta(content: &str) -> Option<ViewportRule> {
         let mut declarations = vec![None; VIEWPORT_DESCRIPTOR_VARIANTS];
diff --git a/tests/unit/style/viewport.rs b/tests/unit/style/viewport.rs
index 2eaefcaff0e..dc15436af5e 100644
--- a/tests/unit/style/viewport.rs
+++ b/tests/unit/style/viewport.rs
@@ -8,7 +8,7 @@ use media_queries::CSSErrorReporterTest;
 use servo_config::prefs::{PREFS, PrefValue};
 use servo_url::ServoUrl;
 use style::media_queries::{Device, MediaType};
-use style::parser::ParserContext;
+use style::parser::{Parse, ParserContext};
 use style::shared_lock::SharedRwLock;
 use style::stylesheets::{Stylesheet, Origin};
 use style::values::specified::LengthOrPercentageOrAuto::{self, Auto};
@@ -294,7 +294,7 @@ fn constrain_viewport() {
 
     macro_rules! from_css {
         ($css:expr) => {
-            &ViewportRule::parse(&mut Parser::new($css), &context).unwrap()
+            &ViewportRule::parse(&context, &mut Parser::new($css)).unwrap()
         }
     }