--- a/servo/components/style/values/specified/mod.rs
+++ b/servo/components/style/values/specified/mod.rs
@@ -558,17 +558,24 @@ impl Parse for Opacity {
     }
 }
 
 impl ToComputedValue for Opacity {
     type ComputedValue = CSSFloat;
 
     #[inline]
     fn to_computed_value(&self, context: &Context) -> CSSFloat {
-        self.0.to_computed_value(context).min(1.0).max(0.0)
+        let value = self.0.to_computed_value(context);
+        if context.for_smil_animation {
+            // SMIL expects to be able to interpolate between out-of-range
+            // opacity values.
+            value
+        } else {
+            value.min(1.0).max(0.0)
+        }
     }
 
     #[inline]
     fn from_computed_value(computed: &CSSFloat) -> Self {
         Opacity(Number::from_computed_value(computed))
     }
 }