diff --git a/components/style/properties/data.py b/components/style/properties/data.py
index 0171c8710b0..3d14e5948f3 100644
--- a/components/style/properties/data.py
+++ b/components/style/properties/data.py
@@ -15,7 +15,7 @@ ALL_SIZES = [(size, False) for size in PHYSICAL_SIZES] + [(size, True) for size
 
 SYSTEM_FONT_LONGHANDS = """font_family font_size font_style
                            font_variant_caps font_stretch font_kerning
-                           font_variant_position font_weight""".split()
+                           font_variant_position font_weight font_size_adjust""".split()
 
 def maybe_moz_logical_alias(product, side, prop):
     if product == "gecko" and side[1]:
diff --git a/components/style/properties/gecko.mako.rs b/components/style/properties/gecko.mako.rs
index 39d16f07522..8b573f2e43d 100644
--- a/components/style/properties/gecko.mako.rs
+++ b/components/style/properties/gecko.mako.rs
@@ -1564,10 +1564,7 @@ fn static_assert() {
     pub fn clone_font_size_adjust(&self) -> longhands::font_size_adjust::computed_value::T {
         use properties::longhands::font_size_adjust::computed_value::T;
 
-        match self.gecko.mFont.sizeAdjust {
-            -1.0 => T::None,
-            _ => T::Number(self.gecko.mFont.sizeAdjust),
-        }
+        T::from_gecko_adjust(self.gecko.mFont.sizeAdjust)
     }
 
     #[allow(non_snake_case)]
diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index 37d97e32fb2..1e8b5dfb453 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -890,6 +890,7 @@ ${helpers.single_keyword_system("font-variant-caps",
 
 <%helpers:longhand products="gecko" name="font-size-adjust" animation_type="normal"
                    spec="https://drafts.csswg.org/css-fonts/#propdef-font-size-adjust">
+    use properties::longhands::system_font::SystemFont;
     use std::fmt;
     use style_traits::ToCss;
     use values::HasViewportPercentage;
@@ -901,6 +902,7 @@ ${helpers.single_keyword_system("font-variant-caps",
     pub enum SpecifiedValue {
         None,
         Number(specified::Number),
+        System(SystemFont),
     }
 
     impl ToCss for SpecifiedValue {
@@ -910,6 +912,7 @@ ${helpers.single_keyword_system("font-variant-caps",
             match *self {
                 SpecifiedValue::None => dest.write_str("none"),
                 SpecifiedValue::Number(number) => number.to_css(dest),
+                SpecifiedValue::System(_) => Ok(()),
             }
         }
     }
@@ -921,6 +924,11 @@ ${helpers.single_keyword_system("font-variant-caps",
             match *self {
                 SpecifiedValue::None => computed_value::T::None,
                 SpecifiedValue::Number(ref n) => computed_value::T::Number(n.to_computed_value(context)),
+                SpecifiedValue::System(_) => {
+                    <%self:nongecko_unreachable>
+                        context.style.cached_system_font.as_ref().unwrap().font_size_adjust
+                    </%self:nongecko_unreachable>
+                }
             }
         }
 
@@ -932,6 +940,19 @@ ${helpers.single_keyword_system("font-variant-caps",
         }
     }
 
+    impl SpecifiedValue {
+        pub fn system_font(f: SystemFont) -> Self {
+            SpecifiedValue::System(f)
+        }
+        pub fn get_system(&self) -> Option<SystemFont> {
+            if let SpecifiedValue::System(s) = *self {
+                Some(s)
+            } else {
+                None
+            }
+        }
+    }
+
     pub mod computed_value {
         use properties::animated_properties::{ComputeDistance, Interpolate};
         use std::fmt;
@@ -956,6 +977,15 @@ ${helpers.single_keyword_system("font-variant-caps",
             }
         }
 
+        impl T {
+            pub fn from_gecko_adjust(gecko: f32) -> Self {
+                match gecko {
+                    -1.0 => T::None,
+                    _ => T::Number(gecko),
+                }
+            }
+        }
+
         impl Interpolate for T {
             fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
                 match (*self, *other) {
@@ -2001,6 +2031,7 @@ ${helpers.single_keyword("-moz-math-variant",
         use app_units::Au;
         use cssparser::Parser;
         use properties::longhands;
+        use std::hash::{Hash, Hasher};
         use values::computed::{ToComputedValue, Context};
         <%
             system_fonts = """caption icon menu message-box small-caption status-bar
@@ -2017,6 +2048,24 @@ ${helpers.single_keyword("-moz-math-variant",
             % endfor
         }
 
+        // ComputedValues are compared at times
+        // so we need these impls. We don't want to
+        // add Eq to Number (which contains a float)
+        // so instead we have an eq impl which skips the
+        // cached values
+        impl PartialEq for ComputedSystemFont {
+            fn eq(&self, other: &Self) -> bool {
+                self.system_font == other.system_font
+            }
+        }
+        impl Eq for ComputedSystemFont {}
+
+        impl Hash for ComputedSystemFont {
+            fn hash<H: Hasher>(&self, hasher: &mut H) {
+                self.system_font.hash(hasher)
+            }
+        }
+
         impl ToComputedValue for SystemFont {
             type ComputedValue = ComputedSystemFont;
 
@@ -2053,6 +2102,7 @@ ${helpers.single_keyword("-moz-math-variant",
                     font_family: longhands::font_family::computed_value::T(family),
                     font_size: Au(system.size),
                     font_weight: weight,
+                    font_size_adjust: longhands::font_size_adjust::computed_value::T::from_gecko_adjust(system.sizeAdjust),
                     % for kwprop in kw_font_props:
                         ${kwprop}: longhands::${kwprop}::computed_value::T::from_gecko_keyword(
                             system.${to_camel_case_lower(kwprop.replace('font_', ''))} as u32
@@ -2082,7 +2132,7 @@ ${helpers.single_keyword("-moz-math-variant",
             debug_assert!(system == context.style.cached_system_font.as_ref().unwrap().system_font)
         }
 
-        #[derive(Clone, Debug, PartialEq, Eq, Hash)]
+        #[derive(Clone, Debug)]
         pub struct ComputedSystemFont {
             % for name in SYSTEM_FONT_LONGHANDS:
                 pub ${name}: longhands::${name}::computed_value::T,