diff --git a/servo/components/style/properties/longhand/font.mako.rs b/servo/components/style/properties/longhand/font.mako.rs
index 5d849c746cfb..c95ea97c2f7b 100644
--- a/servo/components/style/properties/longhand/font.mako.rs
+++ b/servo/components/style/properties/longhand/font.mako.rs
@@ -859,7 +859,7 @@ ${helpers.single_keyword_system("font-variant-caps",
                     base_size.resolve(context).scale_by(pc.0).into()
                 }
                 SpecifiedValue::Length(LengthOrPercentage::Calc(ref calc)) => {
-                    let calc = calc.to_computed_value_zoomed(context);
+                    let calc = calc.to_computed_value_zoomed(context, base_size);
                     calc.to_used_value(Some(base_size.resolve(context))).unwrap().into()
                 }
                 SpecifiedValue::Keyword(ref key, fraction) => {
diff --git a/servo/components/style/values/computed/length.rs b/servo/components/style/values/computed/length.rs
index d29f5aa87fc8..8c90d38eefa0 100644
--- a/servo/components/style/values/computed/length.rs
+++ b/servo/components/style/values/computed/length.rs
@@ -217,7 +217,8 @@ impl ToCss for CalcLengthOrPercentage {
 
 impl specified::CalcLengthOrPercentage {
     /// Compute the value, zooming any absolute units by the zoom function.
-    fn to_computed_value_with_zoom<F>(&self, context: &Context, zoom_fn: F) -> CalcLengthOrPercentage
+    fn to_computed_value_with_zoom<F>(&self, context: &Context, zoom_fn: F,
+                                      base_size: FontBaseSize) -> CalcLengthOrPercentage
         where F: Fn(Au) -> Au {
         let mut length = Au(0);
 
@@ -239,7 +240,7 @@ impl specified::CalcLengthOrPercentage {
                      self.ex.map(FontRelativeLength::Ex),
                      self.rem.map(FontRelativeLength::Rem)] {
             if let Some(val) = *val {
-                length += val.to_computed_value(context, FontBaseSize::CurrentStyle);
+                length += val.to_computed_value(context, base_size);
             }
         }
 
@@ -251,8 +252,8 @@ impl specified::CalcLengthOrPercentage {
     }
 
     /// Compute font-size or line-height taking into account text-zoom if necessary.
-    pub fn to_computed_value_zoomed(&self, context: &Context) -> CalcLengthOrPercentage {
-        self.to_computed_value_with_zoom(context, |abs| context.maybe_zoom_text(abs.into()).0)
+    pub fn to_computed_value_zoomed(&self, context: &Context, base_size: FontBaseSize) -> CalcLengthOrPercentage {
+        self.to_computed_value_with_zoom(context, |abs| context.maybe_zoom_text(abs.into()).0, base_size)
     }
 }
 
@@ -260,7 +261,8 @@ impl ToComputedValue for specified::CalcLengthOrPercentage {
     type ComputedValue = CalcLengthOrPercentage;
 
     fn to_computed_value(&self, context: &Context) -> CalcLengthOrPercentage {
-        self.to_computed_value_with_zoom(context, |abs| abs)
+        // normal properties don't zoom, and compute em units against the current style's font-size
+        self.to_computed_value_with_zoom(context, |abs| abs, FontBaseSize::CurrentStyle)
     }
 
     #[inline]
diff --git a/servo/components/style/values/specified/length.rs b/servo/components/style/values/specified/length.rs
index 490161a4df1c..c816e5420206 100644
--- a/servo/components/style/values/specified/length.rs
+++ b/servo/components/style/values/specified/length.rs
@@ -80,6 +80,7 @@ impl ToCss for FontRelativeLength {
 }
 
 /// A source to resolve font-relative units against
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub enum FontBaseSize {
     /// Use the font-size of the current element
     CurrentStyle,
diff --git a/servo/components/style/values/specified/text.rs b/servo/components/style/values/specified/text.rs
index 08a59e87ff6e..0f4163c3fdd4 100644
--- a/servo/components/style/values/specified/text.rs
+++ b/servo/components/style/values/specified/text.rs
@@ -112,7 +112,7 @@ impl ToComputedValue for LineHeight {
                             ).into()
                     }
                     LengthOrPercentage::Calc(ref calc) => {
-                        let computed_calc = calc.to_computed_value_zoomed(context);
+                        let computed_calc = calc.to_computed_value_zoomed(context, FontBaseSize::CurrentStyle);
                         let font_relative_length =
                             FontRelativeLength::Em(computed_calc.percentage())
                                 .to_computed_value(