Do not inherit kw font size if it was not used due to MathML

components/style/properties/gecko.mako.rs

@@ -1490,8 +1490,10 @@ fn static_assert() {
     /// This function will also handle scriptminsize and scriptlevel
     /// so should not be called when you just want the font sizes to be copied.
     /// Hence the different name.
+    ///
+    /// Returns true if the inherited keyword size was actually used
     pub fn inherit_font_size_from(&mut self, parent: &Self,
-                                  kw_inherited_size: Option<Au>) {
+                                  kw_inherited_size: Option<Au>) -> bool {
         let (adjusted_size, adjusted_unconstrained_size)
             = self.calculate_script_level_size(parent);
         if adjusted_size.0 != parent.gecko.mSize ||
@@ -1513,18 +1515,21 @@ fn static_assert() {
             self.gecko.mFont.size = adjusted_size.0;
             self.gecko.mSize = adjusted_size.0;
             self.gecko.mScriptUnconstrainedSize = adjusted_unconstrained_size.0;
+            false
         } else if let Some(size) = kw_inherited_size {
             // Parent element was a keyword-derived size.
             self.gecko.mFont.size = size.0;
             self.gecko.mSize = size.0;
             // MathML constraints didn't apply here, so we can ignore this.
             self.gecko.mScriptUnconstrainedSize = size.0;
+            true
         } else {
             // MathML isn't affecting us, and our parent element does not
             // have a keyword-derived size. Set things normally.
             self.gecko.mFont.size = parent.gecko.mFont.size;
             self.gecko.mSize = parent.gecko.mSize;
             self.gecko.mScriptUnconstrainedSize = parent.gecko.mScriptUnconstrainedSize;
+            false
         }
     }
 

components/style/properties/longhand/font.mako.rs

@@ -927,10 +927,14 @@ ${helpers.single_keyword_system("font-variant-caps",
         let kw_inherited_size = context.style().font_size_keyword.map(|(kw, ratio)| {
             SpecifiedValue::Keyword(kw, ratio).to_computed_value(context)
         });
-        context.mutate_style().mutate_font()
+        let used_kw = context.mutate_style().mutate_font()
                .inherit_font_size_from(parent, kw_inherited_size);
-        context.mutate_style().font_size_keyword =
-            context.inherited_style.font_size_keyword;
+        if used_kw {
+            context.mutate_style().font_size_keyword =
+                context.inherited_style.font_size_keyword;
+        } else {
+            context.mutate_style().font_size_keyword = None;
+        }
     }
 
     pub fn cascade_initial_font_size(context: &mut Context) {

components/style/properties/properties.mako.rs

@@ -1538,8 +1538,9 @@ pub mod style_structs {
 
                 /// (Servo does not handle MathML, so this just calls copy_font_size_from)
                 pub fn inherit_font_size_from(&mut self, parent: &Self,
-                                              _: Option<Au>) {
+                                              _: Option<Au>) -> bool {
                     self.copy_font_size_from(parent);
+                    false
                 }
                 /// (Servo does not handle MathML, so this just calls set_font_size)
                 pub fn apply_font_size(&mut self,