diff --git a/components/style/properties/shorthand/position.mako.rs b/components/style/properties/shorthand/position.mako.rs
index 2c58895c9d0..49c7c54adbf 100644
--- a/components/style/properties/shorthand/position.mako.rs
+++ b/components/style/properties/shorthand/position.mako.rs
@@ -637,19 +637,22 @@
 
         let justify_content = match justify_content {
             Ok(v) => v,
-            Err(err) => {
-                if !align_content.is_valid_on_both_axes() {
-                    return Err(err);
+            Err(..) => {
+                // https://drafts.csswg.org/css-align-3/#place-content:
+                //
+                //   The second value is assigned to justify-content; if
+                //   omitted, it is copied from the first value, unless that
+                //   value is a <baseline-position> in which case it is
+                //   defaulted to start.
+                //
+                if !align_content.is_baseline_position() {
+                    align_content
+                } else {
+                    ContentDistribution::start()
                 }
-
-                align_content
             }
         };
 
-        if align_content.has_extra_flags() || justify_content.has_extra_flags() {
-            return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
-        }
-
         Ok(expanded! {
             align_content: AlignContent(align_content),
             justify_content: JustifyContent(justify_content),
@@ -682,18 +685,12 @@
 
         let justify = match justify {
             Ok(v) => v,
-            Err(err) => {
-                if !align.is_valid_on_both_axes() {
-                    return Err(err);
-                }
+            Err(..) => {
+                debug_assert!(align.is_valid_on_both_axes());
                 align
             }
         };
 
-        if justify.has_extra_flags() || align.has_extra_flags() {
-            return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
-        }
-
         Ok(expanded! {
             align_self: AlignSelf(align),
             justify_self: JustifySelf(justify),
@@ -729,14 +726,9 @@
         input: &mut Parser<'i, 't>,
     ) -> Result<Longhands, ParseError<'i>> {
         let align = AlignItems::parse(context, input)?;
-        if align.has_extra_flags() {
-            return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
-        }
-        let justify = input.try(|input| JustifyItems::parse(context, input))
-                           .unwrap_or(JustifyItems::from(align));
-        if justify.has_extra_flags() {
-            return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
-        }
+        let justify =
+            input.try(|input| JustifyItems::parse(context, input))
+                 .unwrap_or_else(|_| JustifyItems::from(align));
 
         Ok(expanded! {
             align_items: align,
@@ -746,11 +738,6 @@
 
     impl<'a> ToCss for LonghandsToSerialize<'a> {
         fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: fmt::Write {
-            if self.align_items.has_extra_flags() ||
-               self.justify_items.has_extra_flags() {
-                return Ok(());
-            }
-
             self.align_items.to_css(dest)?;
             if self.align_items.0 != self.justify_items.0 {
                 dest.write_str(" ")?;
diff --git a/components/style/values/specified/align.rs b/components/style/values/specified/align.rs
index 7803388954d..0a87abecf94 100644
--- a/components/style/values/specified/align.rs
+++ b/components/style/values/specified/align.rs
@@ -139,6 +139,12 @@ impl ContentDistribution {
         Self::new(AlignFlags::NORMAL)
     }
 
+    /// `start`
+    #[inline]
+    pub fn start() -> Self {
+        Self::new(AlignFlags::START)
+    }
+
     /// The initial value 'normal'
     #[inline]
     pub fn new(primary: AlignFlags) -> Self {
@@ -155,19 +161,12 @@ impl ContentDistribution {
         self.primary.bits() as u16
     }
 
-    /// Returns whether this value is valid for both axis directions.
-    pub fn is_valid_on_both_axes(&self) -> bool {
-        match self.primary.value() {
-            // <baseline-position> is only allowed on the block axis.
-            AlignFlags::BASELINE |
-            AlignFlags::LAST_BASELINE => false,
-
-            // left | right are only allowed on the inline axis.
-            AlignFlags::LEFT |
-            AlignFlags::RIGHT => false,
-
-            _ => true,
-        }
+    /// Returns whether this value is a <baseline-position>.
+    pub fn is_baseline_position(&self) -> bool {
+        matches!(
+            self.primary.value(),
+            AlignFlags::BASELINE | AlignFlags::LAST_BASELINE
+        )
     }
 
     /// The primary alignment
@@ -176,12 +175,6 @@ impl ContentDistribution {
         self.primary
     }
 
-    /// Whether this value has extra flags.
-    #[inline]
-    pub fn has_extra_flags(self) -> bool {
-        self.primary().intersects(AlignFlags::FLAG_BITS)
-    }
-
     /// Parse a value for align-content / justify-content.
     pub fn parse<'i, 't>(
         input: &mut Parser<'i, 't>,
@@ -309,12 +302,6 @@ impl SelfAlignment {
         }
     }
 
-    /// Whether this value has extra flags.
-    #[inline]
-    pub fn has_extra_flags(self) -> bool {
-        self.0.intersects(AlignFlags::FLAG_BITS)
-    }
-
     /// Parse a self-alignment value on one of the axis.
     pub fn parse<'i, 't>(
         input: &mut Parser<'i, 't>,
@@ -402,12 +389,6 @@ impl AlignItems {
     pub fn normal() -> Self {
         AlignItems(AlignFlags::NORMAL)
     }
-
-    /// Whether this value has extra flags.
-    #[inline]
-    pub fn has_extra_flags(self) -> bool {
-        self.0.intersects(AlignFlags::FLAG_BITS)
-    }
 }
 
 
@@ -449,12 +430,6 @@ impl JustifyItems {
     pub fn normal() -> Self {
         JustifyItems(AlignFlags::NORMAL)
     }
-
-    /// Whether this value has extra flags.
-    #[inline]
-    pub fn has_extra_flags(self) -> bool {
-        self.0.intersects(AlignFlags::FLAG_BITS)
-    }
 }
 
 impl Parse for JustifyItems {