diff --git a/components/style/build_gecko.rs b/components/style/build_gecko.rs
index 7ca5530e9a3..b485efa48ee 100644
--- a/components/style/build_gecko.rs
+++ b/components/style/build_gecko.rs
@@ -293,6 +293,7 @@ mod bindings {
             "BORDER_STYLE_.*",
             "mozilla::SERVO_PREF_.*",
             "kNameSpaceID_.*",
+            "kGenericFont_.*",
         ];
         let whitelist = [
             "RawGecko.*",
diff --git a/components/style/gecko_bindings/bindings.rs b/components/style/gecko_bindings/bindings.rs
index 360766befcc..7efc2252a00 100644
--- a/components/style/gecko_bindings/bindings.rs
+++ b/components/style/gecko_bindings/bindings.rs
@@ -1012,6 +1012,12 @@ extern "C" {
     pub fn Gecko_nsStyleFont_CopyLangFrom(aFont: *mut nsStyleFont,
                                           aSource: *const nsStyleFont);
 }
+extern "C" {
+    pub fn Gecko_nsStyleFont_GetBaseSize(font: *const nsStyleFont,
+                                         pres_context:
+                                             RawGeckoPresContextBorrowed)
+     -> nscoord;
+}
 extern "C" {
     pub fn Gecko_GetMediaFeatures() -> *const nsMediaFeature;
 }
diff --git a/components/style/gecko_bindings/structs_debug.rs b/components/style/gecko_bindings/structs_debug.rs
index d6be67cdf8a..4dfd92f3dfe 100644
--- a/components/style/gecko_bindings/structs_debug.rs
+++ b/components/style/gecko_bindings/structs_debug.rs
@@ -19097,6 +19097,14 @@ pub mod root {
     #[repr(C)]
     #[derive(Debug, Copy, Clone)]
     pub struct gfxFontStyle([u8; 0]);
+    pub const kGenericFont_NONE: u8 = 0;
+    pub const kGenericFont_moz_variable: u8 = 0;
+    pub const kGenericFont_moz_fixed: u8 = 1;
+    pub const kGenericFont_serif: u8 = 2;
+    pub const kGenericFont_sans_serif: u8 = 4;
+    pub const kGenericFont_monospace: u8 = 8;
+    pub const kGenericFont_cursive: u8 = 16;
+    pub const kGenericFont_fantasy: u8 = 32;
     #[repr(C)]
     #[derive(Debug)]
     pub struct nsFont {
diff --git a/components/style/gecko_bindings/structs_release.rs b/components/style/gecko_bindings/structs_release.rs
index 53d42fdde00..661b28a18f9 100644
--- a/components/style/gecko_bindings/structs_release.rs
+++ b/components/style/gecko_bindings/structs_release.rs
@@ -18546,6 +18546,14 @@ pub mod root {
     #[repr(C)]
     #[derive(Debug, Copy, Clone)]
     pub struct gfxFontStyle([u8; 0]);
+    pub const kGenericFont_NONE: u8 = 0;
+    pub const kGenericFont_moz_variable: u8 = 0;
+    pub const kGenericFont_moz_fixed: u8 = 1;
+    pub const kGenericFont_serif: u8 = 2;
+    pub const kGenericFont_sans_serif: u8 = 4;
+    pub const kGenericFont_monospace: u8 = 8;
+    pub const kGenericFont_cursive: u8 = 16;
+    pub const kGenericFont_fantasy: u8 = 32;
     #[repr(C)]
     #[derive(Debug)]
     pub struct nsFont {
diff --git a/components/style/properties/gecko.mako.rs b/components/style/properties/gecko.mako.rs
index 82203d96e19..f8f25da8ed5 100644
--- a/components/style/properties/gecko.mako.rs
+++ b/components/style/properties/gecko.mako.rs
@@ -194,6 +194,11 @@ impl ComputedValues {
 pub struct ${style_struct.gecko_struct_name} {
     gecko: ${style_struct.gecko_ffi_name},
 }
+impl ${style_struct.gecko_struct_name} {
+    pub fn gecko(&self) -> &${style_struct.gecko_ffi_name} {
+        &self.gecko
+    }
+}
 </%def>
 
 <%def name="impl_simple_setter(ident, gecko_ffi_name)">
@@ -1191,14 +1196,31 @@ fn static_assert() {
                     unsafe { Gecko_FontFamilyList_AppendNamed(list, name.0.as_ptr()); }
                 }
                 FontFamily::Generic(ref name) => {
-                    let family_type =
-                        if name == &atom!("serif") { FontFamilyType::eFamily_serif }
-                        else if name == &atom!("sans-serif") { FontFamilyType::eFamily_sans_serif }
-                        else if name == &atom!("cursive") { FontFamilyType::eFamily_cursive }
-                        else if name == &atom!("fantasy") { FontFamilyType::eFamily_fantasy }
-                        else if name == &atom!("monospace") { FontFamilyType::eFamily_monospace }
-                        else if name == &atom!("-moz-fixed") { FontFamilyType::eFamily_moz_fixed }
-                        else { panic!("Unknown generic font family") };
+                    let (family_type, generic) =
+                        if name == &atom!("serif") {
+                            (FontFamilyType::eFamily_serif,
+                             structs::kGenericFont_serif)
+                        } else if name == &atom!("sans-serif") {
+                            (FontFamilyType::eFamily_sans_serif,
+                             structs::kGenericFont_sans_serif)
+                        } else if name == &atom!("cursive") {
+                            (FontFamilyType::eFamily_cursive,
+                             structs::kGenericFont_cursive)
+                        } else if name == &atom!("fantasy") {
+                            (FontFamilyType::eFamily_fantasy,
+                             structs::kGenericFont_fantasy)
+                        } else if name == &atom!("monospace") {
+                            (FontFamilyType::eFamily_monospace,
+                             structs::kGenericFont_monospace)
+                        } else if name == &atom!("-moz-fixed") {
+                            (FontFamilyType::eFamily_moz_fixed,
+                             structs::kGenericFont_moz_fixed)
+                        } else {
+                            panic!("Unknown generic font family")
+                        };
+                    if v.0.len() == 1 {
+                        self.gecko.mGenericID = generic;
+                    }
                     unsafe { Gecko_FontFamilyList_AppendGeneric(list, family_type); }
                 }
             }
diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index 49b3557c8cb..7b1e829eaf0 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -482,29 +482,79 @@ ${helpers.single_keyword("font-variant-caps",
         }
     }
 
-    impl ToComputedValue for KeywordSize {
-        type ComputedValue = Au;
-        #[inline]
-        fn to_computed_value(&self, _: &Context) -> computed_value::T {
-            // https://drafts.csswg.org/css-fonts-3/#font-size-prop
-            use values::FONT_MEDIUM_PX;
-            match *self {
-                XXSmall => Au::from_px(FONT_MEDIUM_PX) * 3 / 5,
-                XSmall => Au::from_px(FONT_MEDIUM_PX) * 3 / 4,
-                Small => Au::from_px(FONT_MEDIUM_PX) * 8 / 9,
-                Medium => Au::from_px(FONT_MEDIUM_PX),
-                Large => Au::from_px(FONT_MEDIUM_PX) * 6 / 5,
-                XLarge => Au::from_px(FONT_MEDIUM_PX) * 3 / 2,
-                XXLarge => Au::from_px(FONT_MEDIUM_PX) * 2,
-                XXXLarge => Au::from_px(FONT_MEDIUM_PX) * 3,
+    % if product == "servo":
+        impl ToComputedValue for KeywordSize {
+            type ComputedValue = Au;
+            #[inline]
+            fn to_computed_value(&self, _: &Context) -> computed_value::T {
+                // https://drafts.csswg.org/css-fonts-3/#font-size-prop
+                use values::FONT_MEDIUM_PX;
+                match *self {
+                    XXSmall => Au::from_px(FONT_MEDIUM_PX) * 3 / 5,
+                    XSmall => Au::from_px(FONT_MEDIUM_PX) * 3 / 4,
+                    Small => Au::from_px(FONT_MEDIUM_PX) * 8 / 9,
+                    Medium => Au::from_px(FONT_MEDIUM_PX),
+                    Large => Au::from_px(FONT_MEDIUM_PX) * 6 / 5,
+                    XLarge => Au::from_px(FONT_MEDIUM_PX) * 3 / 2,
+                    XXLarge => Au::from_px(FONT_MEDIUM_PX) * 2,
+                    XXXLarge => Au::from_px(FONT_MEDIUM_PX) * 3,
+                }
+            }
+
+            #[inline]
+            fn from_computed_value(_: &computed_value::T) -> Self {
+                unreachable!()
             }
         }
+    % else:
+        impl ToComputedValue for KeywordSize {
+            type ComputedValue = Au;
+            #[inline]
+            fn to_computed_value(&self, cx: &Context) -> computed_value::T {
+                use gecko_bindings::bindings::Gecko_nsStyleFont_GetBaseSize;
+                use values::specified::length::au_to_int_px;
+                // Data from nsRuleNode.cpp in Gecko
+                // Mapping from base size and HTML size to pixels
+                // The first index is (base_size - 9), the second is the
+                // HTML size. "0" is CSS keyword xx-small, not HTML size 0,
+                // since HTML size 0 is the same as 1.
+                //
+                //  xxs   xs      s      m     l      xl     xxl   -
+                //  -     0/1     2      3     4      5      6     7
+                static FONT_SIZE_MAPPING: [[i32; 8]; 8] = [
+                    [9,    9,     9,     9,    11,    14,    18,    27],
+                    [9,    9,     9,    10,    12,    15,    20,    30],
+                    [9,    9,    10,    11,    13,    17,    22,    33],
+                    [9,    9,    10,    12,    14,    18,    24,    36],
+                    [9,   10,    12,    13,    16,    20,    26,    39],
+                    [9,   10,    12,    14,    17,    21,    28,    42],
+                    [9,   10,    13,    15,    18,    23,    30,    45],
+                    [9,   10,    13,    16,    18,    24,    32,    48]
+                ];
 
-        #[inline]
-        fn from_computed_value(_: &computed_value::T) -> Self {
-            unreachable!()
+                static FONT_SIZE_FACTORS: [i32; 8] = [60, 75, 89, 100, 120, 150, 200, 300];
+
+                // XXXManishearth handle quirks mode
+
+                let base_size = unsafe {
+                    Gecko_nsStyleFont_GetBaseSize(cx.style().get_font().gecko(),
+                                                  &*cx.device.pres_context)
+                };
+                let base_size_px = au_to_int_px(base_size as f32);
+                let html_size = *self as usize;
+                if base_size_px >= 9 && base_size_px <= 16 {
+                    Au::from_px(FONT_SIZE_MAPPING[(base_size_px - 9) as usize][html_size])
+                } else {
+                    Au(FONT_SIZE_FACTORS[html_size] * base_size / 100)
+                }
+            }
+
+            #[inline]
+            fn from_computed_value(_: &computed_value::T) -> Self {
+                unreachable!()
+            }
         }
-    }
+    % endif
 
     impl SpecifiedValue {
         /// https://html.spec.whatwg.org/multipage/#rules-for-parsing-a-legacy-font-size
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index bffae92c4ff..7a16ce41d8f 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -33,6 +33,14 @@ const AU_PER_Q: CSSFloat = AU_PER_MM / 4.;
 const AU_PER_PT: CSSFloat = AU_PER_IN / 72.;
 const AU_PER_PC: CSSFloat = AU_PER_PT * 12.;
 
+/// Same as Gecko's AppUnitsToIntCSSPixels
+///
+/// Converts app units to integer pixel values,
+/// rounding during the conversion
+pub fn au_to_int_px(au: f32) -> i32 {
+    (au / AU_PER_PX).round() as i32
+}
+
 #[derive(Clone, PartialEq, Copy, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 /// A font relative length.