diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index df95d9f464c..c400f513bdc 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -418,6 +418,7 @@ ${helpers.single_keyword("font-variant-caps",
     use values::{FONT_MEDIUM_PX, HasViewportPercentage};
     use values::specified::{FontRelativeLength, LengthOrPercentage, Length};
     use values::specified::{NoCalcLength, Percentage};
+    use values::specified::length::FontBaseSize;
 
     impl ToCss for SpecifiedValue {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
@@ -627,6 +628,43 @@ ${helpers.single_keyword("font-variant-caps",
             }
             None
         }
+
+        /// Compute it against a given base font size
+        pub fn to_computed_value_against(&self, context: &Context, base_size: FontBaseSize) -> Au {
+            use values::specified::length::FontRelativeLength;
+            match *self {
+                SpecifiedValue::Length(LengthOrPercentage::Length(
+                        NoCalcLength::FontRelative(value))) => {
+                    value.to_computed_value(context, base_size)
+                }
+                SpecifiedValue::Length(LengthOrPercentage::Length(
+                        NoCalcLength::ServoCharacterWidth(value))) => {
+                    value.to_computed_value(base_size.resolve(context))
+                }
+                SpecifiedValue::Length(LengthOrPercentage::Length(ref l)) => {
+                    l.to_computed_value(context)
+                }
+                SpecifiedValue::Length(LengthOrPercentage::Percentage(Percentage(value))) => {
+                    base_size.resolve(context).scale_by(value)
+                }
+                SpecifiedValue::Length(LengthOrPercentage::Calc(ref calc)) => {
+                    let calc = calc.to_computed_value(context);
+                    calc.length() +base_size.resolve(context)
+                                           .scale_by(calc.percentage())
+                }
+                SpecifiedValue::Keyword(ref key, fraction) => {
+                    key.to_computed_value(context).scale_by(fraction)
+                }
+                SpecifiedValue::Smaller => {
+                    FontRelativeLength::Em(0.85)
+                        .to_computed_value(context, base_size)
+                }
+                SpecifiedValue::Larger => {
+                    FontRelativeLength::Em(1.2)
+                        .to_computed_value(context, base_size)
+                }
+            }
+        }
     }
 
     #[inline]
@@ -640,44 +678,13 @@ ${helpers.single_keyword("font-variant-caps",
         SpecifiedValue::Keyword(Medium, 1.)
     }
 
+
     impl ToComputedValue for SpecifiedValue {
         type ComputedValue = computed_value::T;
 
         #[inline]
         fn to_computed_value(&self, context: &Context) -> computed_value::T {
-            use values::specified::length::FontRelativeLength;
-            match *self {
-                SpecifiedValue::Length(LengthOrPercentage::Length(
-                        NoCalcLength::FontRelative(value))) => {
-                    value.to_computed_value(context, /* use inherited */ true)
-                }
-                SpecifiedValue::Length(LengthOrPercentage::Length(
-                        NoCalcLength::ServoCharacterWidth(value))) => {
-                    value.to_computed_value(context.inherited_style().get_font().clone_font_size())
-                }
-                SpecifiedValue::Length(LengthOrPercentage::Length(ref l)) => {
-                    l.to_computed_value(context)
-                }
-                SpecifiedValue::Length(LengthOrPercentage::Percentage(Percentage(value))) => {
-                    context.inherited_style().get_font().clone_font_size().scale_by(value)
-                }
-                SpecifiedValue::Length(LengthOrPercentage::Calc(ref calc)) => {
-                    let calc = calc.to_computed_value(context);
-                    calc.length() + context.inherited_style().get_font().clone_font_size()
-                                           .scale_by(calc.percentage())
-                }
-                SpecifiedValue::Keyword(ref key, fraction) => {
-                    key.to_computed_value(context).scale_by(fraction)
-                }
-                SpecifiedValue::Smaller => {
-                    FontRelativeLength::Em(0.85).to_computed_value(context,
-                                                                   /* use_inherited */ true)
-                }
-                SpecifiedValue::Larger => {
-                    FontRelativeLength::Em(1.2).to_computed_value(context,
-                                                                   /* use_inherited */ true)
-                }
-            }
+            self.to_computed_value_against(context, FontBaseSize::InheritedStyle)
         }
 
         #[inline]
diff --git a/components/style/values/computed/length.rs b/components/style/values/computed/length.rs
index c21a03c1ca6..9332b03162a 100644
--- a/components/style/values/computed/length.rs
+++ b/components/style/values/computed/length.rs
@@ -10,7 +10,7 @@ use std::fmt;
 use style_traits::ToCss;
 use super::{Number, ToComputedValue, Context};
 use values::{Auto, CSSFloat, Either, ExtremumLength, None_, Normal, specified};
-use values::specified::length::{AbsoluteLength, FontRelativeLength, ViewportPercentageLength};
+use values::specified::length::{AbsoluteLength, FontBaseSize, FontRelativeLength, ViewportPercentageLength};
 
 pub use super::image::{EndingShape as GradientShape, Gradient, GradientKind, Image};
 pub use super::image::{LengthOrKeyword, LengthOrPercentageOrKeyword};
@@ -25,7 +25,7 @@ impl ToComputedValue for specified::NoCalcLength {
             specified::NoCalcLength::Absolute(length) =>
                 length.to_computed_value(context),
             specified::NoCalcLength::FontRelative(length) =>
-                length.to_computed_value(context, /* use inherited */ false),
+                length.to_computed_value(context, /* base_size */ FontBaseSize::CurrentStyle),
             specified::NoCalcLength::ViewportPercentage(length) =>
                 length.to_computed_value(context.viewport_size()),
             specified::NoCalcLength::ServoCharacterWidth(length) =>
@@ -139,6 +139,7 @@ impl ToComputedValue for specified::CalcLengthOrPercentage {
     type ComputedValue = CalcLengthOrPercentage;
 
     fn to_computed_value(&self, context: &Context) -> CalcLengthOrPercentage {
+
         let mut length = Au(0);
 
         if let Some(absolute) = self.absolute {
@@ -159,7 +160,7 @@ impl ToComputedValue for specified::CalcLengthOrPercentage {
                      self.ex.map(FontRelativeLength::Ex),
                      self.rem.map(FontRelativeLength::Rem)] {
             if let Some(val) = *val {
-                length += val.to_computed_value(context, /* use inherited */ false);
+                length += val.to_computed_value(context, /* base_size */ FontBaseSize::CurrentStyle);
             }
         }
 
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 4d41be3e07e..581cd74e3e5 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -76,10 +76,31 @@ impl ToCss for FontRelativeLength {
     }
 }
 
+/// A source to resolve font-relative units against
+pub enum FontBaseSize {
+    /// Use the font-size of the current element
+    CurrentStyle,
+    /// Use the inherited font-size
+    InheritedStyle,
+    /// Use a custom base size
+    Custom(Au),
+}
+
+impl FontBaseSize {
+    /// Calculate the actual size for a given context
+    pub fn resolve(&self, context: &Context) -> Au {
+        match *self {
+            FontBaseSize::Custom(size) => size,
+            FontBaseSize::CurrentStyle => context.style().get_font().clone_font_size(),
+            FontBaseSize::InheritedStyle => context.inherited_style().get_font().clone_font_size(),
+        }
+    }
+}
+
 impl FontRelativeLength {
-    /// Computes the font-relative length. We use the use_inherited flag to
-    /// special-case the computation of font-size.
-    pub fn to_computed_value(&self, context: &Context, use_inherited: bool) -> Au {
+    /// Computes the font-relative length. We use the inherited_size
+    /// flag to pass a different size for computing font-size and unconstrained font-size
+    pub fn to_computed_value(&self, context: &Context, base_size: FontBaseSize) -> Au {
         fn query_font_metrics(context: &Context, reference_font_size: Au) -> FontMetricsQueryResult {
             context.font_metrics_provider.query(context.style().get_font(),
                                                 reference_font_size,
@@ -88,11 +109,7 @@ impl FontRelativeLength {
                                                 context.device)
         }
 
-        let reference_font_size = if use_inherited {
-            context.inherited_style().get_font().clone_font_size()
-        } else {
-            context.style().get_font().clone_font_size()
-        };
+        let reference_font_size = base_size.resolve(context);
 
         let root_font_size = context.style().root_font_size;
         match *self {