diff --git a/components/script/dom/webidls/CSSStyleDeclaration.webidl b/components/script/dom/webidls/CSSStyleDeclaration.webidl
index 3444a460441..014e12740bf 100644
--- a/components/script/dom/webidls/CSSStyleDeclaration.webidl
+++ b/components/script/dom/webidls/CSSStyleDeclaration.webidl
@@ -204,4 +204,6 @@ partial interface CSSStyleDeclaration {
   [SetterThrows, TreatNullAs=EmptyString] attribute DOMString transitionTimingFunction;
   [SetterThrows, TreatNullAs=EmptyString] attribute DOMString transitionProperty;
   [SetterThrows, TreatNullAs=EmptyString] attribute DOMString transitionDelay;
+
+  [SetterThrows, TreatNullAs=EmptyString] attribute DOMString flexDirection;
 };
diff --git a/components/style/properties.mako.rs b/components/style/properties.mako.rs
index ea6ab4a3378..db509fa272e 100644
--- a/components/style/properties.mako.rs
+++ b/components/style/properties.mako.rs
@@ -4746,6 +4746,14 @@ pub mod longhands {
         pub use properties::longhands::transition_duration::{get_initial_single_value};
         pub use properties::longhands::transition_duration::{get_initial_value, parse, parse_one};
     </%self:longhand>
+
+    // CSS Flexible Box Layout Module Level 1
+    // http://www.w3.org/TR/css3-flexbox/
+
+    ${new_style_struct("Flex", is_inherited=False)}
+
+    // Flex container properties
+    ${single_keyword("flex-direction", "row row-reverse column column-reverse", experimental=True)}
 }