--- a/servo/components/style/properties/helpers/animated_properties.mako.rs
+++ b/servo/components/style/properties/helpers/animated_properties.mako.rs
@@ -3076,17 +3076,18 @@ impl From<${ty}ShadowList> for Intermedi
 }
 % endfor
 
 impl From<IntermediateShadow> for Shadow {
     fn from(shadow: IntermediateShadow) -> Shadow {
         Shadow {
             offset_x: shadow.offset_x,
             offset_y: shadow.offset_y,
-            blur_radius: shadow.blur_radius,
+            // blur-radius must be non-negative.
+            blur_radius: shadow.blur_radius.restrict_value(Restrictions::NonNegative),
             spread_radius: shadow.spread_radius,
             color: shadow.color.into(),
             inset: shadow.inset,
         }
     }
 }
 
 impl From<Shadow> for IntermediateShadow {