diff --git a/components/style/properties/data.py b/components/style/properties/data.py
index 4fe90fb560b..d95eed44dd9 100644
--- a/components/style/properties/data.py
+++ b/components/style/properties/data.py
@@ -495,6 +495,7 @@ class Longhand(Property):
                 "Percentage",
                 "PositiveIntegerOrNone",
                 "Resize",
+                "RubyPosition",
                 "SVGOpacity",
                 "SVGPaintOrder",
                 "ScrollSnapAlign",
diff --git a/components/style/properties/longhands/inherited_text.mako.rs b/components/style/properties/longhands/inherited_text.mako.rs
index 0ce27e0e18f..b8b603e8b2e 100644
--- a/components/style/properties/longhands/inherited_text.mako.rs
+++ b/components/style/properties/longhands/inherited_text.mako.rs
@@ -329,13 +329,13 @@ ${helpers.single_keyword(
     spec="https://drafts.csswg.org/css-ruby/#ruby-align-property",
 )}
 
-${helpers.single_keyword(
+${helpers.predefined_type(
     "ruby-position",
-    "over under",
+    "RubyPosition",
+    "Default::default()",
     engines="gecko",
-    animation_value_type="discrete",
     spec="https://drafts.csswg.org/css-ruby/#ruby-position-property",
-    gecko_enum_prefix="StyleRubyPosition",
+    animation_value_type="discrete",
 )}
 
 // CSS Writing Modes Module Level 3
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index 0c9496c2fad..5ed404cf5e3 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -87,7 +87,7 @@ pub use self::svg::{SVGLength, SVGOpacity, SVGPaint, SVGPaintKind};
 pub use self::svg::{SVGPaintOrder, SVGStrokeDashArray, SVGWidth};
 pub use self::text::TextUnderlinePosition;
 pub use self::text::{InitialLetter, LetterSpacing, LineBreak, LineHeight};
-pub use self::text::{OverflowWrap, TextOverflow, WordBreak, WordSpacing};
+pub use self::text::{OverflowWrap, RubyPosition, TextOverflow, WordBreak, WordSpacing};
 pub use self::text::{TextAlign, TextAlignLast, TextEmphasisPosition, TextEmphasisStyle};
 pub use self::text::{TextDecorationLength, TextDecorationSkipInk};
 pub use self::time::Time;
diff --git a/components/style/values/computed/text.rs b/components/style/values/computed/text.rs
index b77695e06c0..4f952fa21cc 100644
--- a/components/style/values/computed/text.rs
+++ b/components/style/values/computed/text.rs
@@ -19,7 +19,7 @@ use std::fmt::{self, Write};
 use style_traits::{CssWriter, ToCss};
 
 pub use crate::values::specified::text::{TextAlignLast, TextUnderlinePosition};
-pub use crate::values::specified::{LineBreak, OverflowWrap, WordBreak};
+pub use crate::values::specified::{LineBreak, OverflowWrap, RubyPosition, WordBreak};
 pub use crate::values::specified::{TextDecorationLine, TextEmphasisPosition};
 pub use crate::values::specified::{TextDecorationSkipInk, TextTransform};
 
diff --git a/components/style/values/specified/mod.rs b/components/style/values/specified/mod.rs
index d91f9b8195f..946bf3c2cc9 100644
--- a/components/style/values/specified/mod.rs
+++ b/components/style/values/specified/mod.rs
@@ -86,6 +86,7 @@ pub use self::svg::{SVGPaintOrder, SVGStrokeDashArray, SVGWidth};
 pub use self::svg_path::SVGPathData;
 pub use self::text::TextAlignLast;
 pub use self::text::TextUnderlinePosition;
+pub use self::text::RubyPosition;
 pub use self::text::{InitialLetter, LetterSpacing, LineBreak, LineHeight, TextAlign};
 pub use self::text::{OverflowWrap, TextEmphasisPosition, TextEmphasisStyle, WordBreak};
 pub use self::text::{TextAlignKeyword, TextDecorationLine, TextOverflow, WordSpacing};
diff --git a/components/style/values/specified/text.rs b/components/style/values/specified/text.rs
index 7c5c712eb3e..3fe3b92c354 100644
--- a/components/style/values/specified/text.rs
+++ b/components/style/values/specified/text.rs
@@ -22,6 +22,7 @@ use selectors::parser::SelectorParseErrorKind;
 use std::fmt::{self, Write};
 use style_traits::values::SequenceWriter;
 use style_traits::{CssWriter, ParseError, StyleParseErrorKind, ToCss};
+use style_traits::{KeywordsCollectFn, SpecifiedValueInfo};
 use unicode_segmentation::UnicodeSegmentation;
 
 /// A specified type for the `initial-letter` property.
@@ -1193,3 +1194,85 @@ impl ToCss for TextUnderlinePosition {
         Ok(())
     }
 }
+
+/// Values for `ruby-position` property
+#[repr(u8)]
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    MallocSizeOf,
+    PartialEq,
+    ToComputedValue,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[allow(missing_docs)]
+pub enum RubyPosition {
+    AlternateOver,
+    AlternateUnder,
+    Over,
+    Under,
+}
+
+impl Default for RubyPosition {
+    fn default() -> Self {
+        if static_prefs::pref!("layout.css.ruby.position-alternate.enabled") {
+            RubyPosition::AlternateOver
+        } else {
+            RubyPosition::Over
+        }
+    }
+}
+
+impl Parse for RubyPosition {
+    fn parse<'i, 't>(
+        _context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<RubyPosition, ParseError<'i>> {
+        // Parse alternate before
+        let alternate_enabled = static_prefs::pref!("layout.css.ruby.position-alternate.enabled");
+        let alternate = alternate_enabled &&
+            input.try_parse(|i| i.expect_ident_matching("alternate")).is_ok();
+        if alternate && input.is_exhausted() {
+            return Ok(RubyPosition::AlternateOver);
+        }
+        // Parse over / under
+        let over = try_match_ident_ignore_ascii_case! { input,
+            "over" => true,
+            "under" => false,
+        };
+        // Parse alternate after
+        let alternate = alternate ||
+            (alternate_enabled &&
+             input.try_parse(|i| i.expect_ident_matching("alternate")).is_ok());
+
+        Ok(match (over, alternate) {
+            (true, true) => RubyPosition::AlternateOver,
+            (false, true) => RubyPosition::AlternateUnder,
+            (true, false) => RubyPosition::Over,
+            (false, false) => RubyPosition::Under,
+        })
+    }
+}
+
+impl ToCss for RubyPosition {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        dest.write_str(match self {
+            RubyPosition::AlternateOver => "alternate",
+            RubyPosition::AlternateUnder => "alternate under",
+            RubyPosition::Over => "over",
+            RubyPosition::Under => "under",
+        })
+    }
+}
+
+impl SpecifiedValueInfo for RubyPosition {
+    fn collect_completion_keywords(f: KeywordsCollectFn) {
+        f(&["alternate", "over", "under"])
+    }
+}