diff --git a/components/style/properties/longhands/box.mako.rs b/components/style/properties/longhands/box.mako.rs
index a68b179c4c0..b8b6cbc8c34 100644
--- a/components/style/properties/longhands/box.mako.rs
+++ b/components/style/properties/longhands/box.mako.rs
@@ -63,6 +63,7 @@ ${helpers.predefined_type(
     "Float",
     "computed::Float::None",
     engines="gecko servo-2013 servo-2020",
+    servo_2020_pref="layout.2020.unimplemented",
     initial_specified_value="specified::Float::None",
     spec="https://drafts.csswg.org/css-box/#propdef-float",
     animation_value_type="discrete",