--- a/servo/components/style/properties/longhands/position.mako.rs
+++ b/servo/components/style/properties/longhands/position.mako.rs
@@ -67,28 +67,30 @@ macro_rules! impl_align_conversions {
 
 // CSS Flexible Box Layout Module Level 1
 // http://www.w3.org/TR/css3-flexbox/
 
 // Flex container properties
 ${helpers.single_keyword(
     "flex-direction",
     "row row-reverse column column-reverse",
-    engines="gecko servo-2013",
+    engines="gecko servo-2013 servo-2020",
+    servo_2020_pref="layout.flexbox.enabled",
     spec="https://drafts.csswg.org/css-flexbox/#flex-direction-property",
     extra_prefixes="webkit",
     animation_value_type="discrete",
     servo_restyle_damage = "reflow",
     gecko_enum_prefix = "StyleFlexDirection",
 )}
 
 ${helpers.single_keyword(
     "flex-wrap",
     "nowrap wrap wrap-reverse",
-    engines="gecko servo-2013",
+    engines="gecko servo-2013 servo-2020",
+    servo_2020_pref="layout.flexbox.enabled",
     spec="https://drafts.csswg.org/css-flexbox/#flex-wrap-property",
     extra_prefixes="webkit",
     animation_value_type="discrete",
     servo_restyle_damage = "reflow",
     gecko_enum_prefix = "StyleFlexWrap",
 )}
 
 % if engine == "servo-2013":
@@ -200,28 +202,30 @@ macro_rules! impl_align_conversions {
     impl_align_conversions!(crate::values::specified::align::JustifyItems);
 % endif
 
 // Flex item properties
 ${helpers.predefined_type(
     "flex-grow",
     "NonNegativeNumber",
     "From::from(0.0)",
-    engines="gecko servo-2013",
+    engines="gecko servo-2013 servo-2020",
+    servo_2020_pref="layout.flexbox.enabled",
     spec="https://drafts.csswg.org/css-flexbox/#flex-grow-property",
     extra_prefixes="webkit",
     animation_value_type="NonNegativeNumber",
     servo_restyle_damage="reflow",
 )}
 
 ${helpers.predefined_type(
     "flex-shrink",
     "NonNegativeNumber",
     "From::from(1.0)",
-    engines="gecko servo-2013",
+    engines="gecko servo-2013 servo-2020",
+    servo_2020_pref="layout.flexbox.enabled",
     spec="https://drafts.csswg.org/css-flexbox/#flex-shrink-property",
     extra_prefixes="webkit",
     animation_value_type="NonNegativeNumber",
     servo_restyle_damage = "reflow",
 )}
 
 // https://drafts.csswg.org/css-align/#align-self-property
 % if engine in ["servo-2013", "servo-2020"]:
@@ -273,17 +277,18 @@ macro_rules! impl_align_conversions {
     spec="https://drafts.csswg.org/css-flexbox/#order-property",
     servo_restyle_damage="reflow",
 )}
 
 ${helpers.predefined_type(
     "flex-basis",
     "FlexBasis",
     "computed::FlexBasis::auto()",
-    engines="gecko servo-2013",
+    engines="gecko servo-2013 servo-2020",
+    servo_2020_pref="layout.flexbox.enabled",
     spec="https://drafts.csswg.org/css-flexbox/#flex-basis-property",
     extra_prefixes="webkit",
     animation_value_type="FlexBasis",
     servo_restyle_damage="reflow",
     boxed=True,
 )}
 
 % for (size, logical) in ALL_SIZES:

--- a/servo/components/style/properties/shorthands/position.mako.rs
+++ b/servo/components/style/properties/shorthands/position.mako.rs
@@ -1,16 +1,17 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 <%namespace name="helpers" file="/helpers.mako.rs" />
 
 <%helpers:shorthand name="flex-flow"
-                    engines="gecko servo-2013"
+                    engines="gecko servo-2013 servo-2020",
+                    servo_2020_pref="layout.flexbox.enabled",
                     sub_properties="flex-direction flex-wrap"
                     extra_prefixes="webkit"
                     derive_serialize="True"
                     spec="https://drafts.csswg.org/css-flexbox/#flex-flow-property">
     use crate::properties::longhands::{flex_direction, flex_wrap};
 
     pub fn parse_value<'i, 't>(
         context: &ParserContext,
@@ -40,17 +41,18 @@
         Ok(expanded! {
             flex_direction: unwrap_or_initial!(flex_direction, direction),
             flex_wrap: unwrap_or_initial!(flex_wrap, wrap),
         })
     }
 </%helpers:shorthand>
 
 <%helpers:shorthand name="flex"
-                    engines="gecko servo-2013"
+                    engines="gecko servo-2013 servo-2020",
+                    servo_2020_pref="layout.flexbox.enabled",
                     sub_properties="flex-grow flex-shrink flex-basis"
                     extra_prefixes="webkit"
                     derive_serialize="True"
                     spec="https://drafts.csswg.org/css-flexbox/#flex-property">
     use crate::parser::Parse;
     use crate::values::specified::NonNegativeNumber;
     use crate::properties::longhands::flex_basis::SpecifiedValue as FlexBasis;