--- a/servo/components/style/properties/longhand/box.mako.rs
+++ b/servo/components/style/properties/longhand/box.mako.rs
@@ -637,16 +637,26 @@
     "shape-image-threshold", "Opacity", "0.0",
     products="gecko",
     gecko_pref="layout.css.shape-outside.enabled",
     animation_value_type="ComputedValue",
     spec="https://drafts.csswg.org/css-shapes/#shape-image-threshold-property",
 )}
 
 ${helpers.predefined_type(
+    "shape-margin",
+    "NonNegativeLengthOrPercentage",
+    "computed::NonNegativeLengthOrPercentage::zero()",
+    products="gecko",
+    gecko_pref="layout.css.shape-outside.enabled",
+    animation_value_type="NonNegativeLengthOrPercentage",
+    spec="https://drafts.csswg.org/css-shapes/#shape-margin",
+)}
+
+${helpers.predefined_type(
     "shape-outside",
     "basic_shape::FloatAreaShape",
     "generics::basic_shape::ShapeSource::None",
     products="gecko",
     boxed=True,
     gecko_pref="layout.css.shape-outside.enabled",
     animation_value_type="ComputedValue",
     flags="APPLIES_TO_FIRST_LETTER",