diff --git a/components/style/properties/shorthands/background.mako.rs b/components/style/properties/shorthands/background.mako.rs
index af98342bfe692..7986201e423a2 100644
--- a/components/style/properties/shorthands/background.mako.rs
+++ b/components/style/properties/shorthands/background.mako.rs
@@ -15,7 +15,7 @@
     use crate::properties::longhands::background_clip;
     use crate::properties::longhands::background_clip::single_value::computed_value::T as Clip;
     use crate::properties::longhands::background_origin::single_value::computed_value::T as Origin;
-    use crate::values::specified::{Color, Position, PositionComponent};
+    use crate::values::specified::{AllowQuirks, Color, Position, PositionComponent};
     use crate::parser::Parse;
 
     // FIXME(emilio): Should be the same type!
@@ -64,7 +64,9 @@
                     }
                 }
                 if position.is_none() {
-                    if let Ok(value) = input.try(|input| Position::parse(context, input)) {
+                    if let Ok(value) = input.try(|input| {
+                        Position::parse_three_value_quirky(context, input, AllowQuirks::No)
+                    }) {
                         position = Some(value);
 
                         // Parse background size, if applicable.
@@ -211,7 +213,7 @@
         let mut any = false;
 
         input.parse_comma_separated(|input| {
-            let value = Position::parse_quirky(context, input, AllowQuirks::Yes)?;
+            let value = Position::parse_three_value_quirky(context, input, AllowQuirks::Yes)?;
             position_x.push(value.horizontal);
             position_y.push(value.vertical);
             any = true;
diff --git a/components/style/values/specified/position.rs b/components/style/values/specified/position.rs
index 85516790c4f7e..e4ab6536a6532 100644
--- a/components/style/values/specified/position.rs
+++ b/components/style/values/specified/position.rs
@@ -94,13 +94,17 @@ impl Parse for Position {
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
     ) -> Result<Self, ParseError<'i>> {
-        Self::parse_quirky(context, input, AllowQuirks::No)
+        let position = Self::parse_three_value_quirky(context, input, AllowQuirks::No)?;
+        if position.is_three_value_syntax() {
+            return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
+        }
+        Ok(position)
     }
 }
 
 impl Position {
-    /// Parses a `<position>`, with quirks.
-    pub fn parse_quirky<'i, 't>(
+    /// Parses a `<bg-position>`, with quirks.
+    pub fn parse_three_value_quirky<'i, 't>(
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
         allow_quirks: AllowQuirks,
@@ -183,6 +187,12 @@ impl Position {
     pub fn center() -> Self {
         Self::new(PositionComponent::Center, PositionComponent::Center)
     }
+
+    /// Returns true if this uses a 3 value syntax.
+    #[inline]
+    fn is_three_value_syntax(&self) -> bool {
+        self.horizontal.component_count() != self.vertical.component_count()
+    }
 }
 
 impl ToCss for Position {
@@ -252,6 +262,17 @@ impl<S> PositionComponent<S> {
     pub fn zero() -> Self {
         PositionComponent::Length(LengthPercentage::Percentage(Percentage::zero()))
     }
+
+    /// Returns the count of this component.
+    fn component_count(&self) -> usize {
+        match *self {
+            PositionComponent::Length(..) |
+            PositionComponent::Center => 1,
+            PositionComponent::Side(_, ref lp) => {
+                if lp.is_some() { 2 } else { 1 }
+            }
+        }
+    }
 }
 
 impl<S: Side> ToComputedValue for PositionComponent<S> {