--- a/servo/components/style/values/specified/box.rs
+++ b/servo/components/style/values/specified/box.rs
@@ -29,16 +29,27 @@ fn moz_display_values_enabled(context: &
 }
 
 #[cfg(feature = "gecko")]
 fn moz_box_display_values_enabled(context: &ParserContext) -> bool {
     context.in_ua_or_chrome_sheet() ||
         static_prefs::pref!("layout.css.xul-box-display-values.content.enabled")
 }
 
+fn flexbox_enabled() -> bool {
+    if cfg!(feature = "servo-layout-2020") {
+        servo_config::prefs::pref_map()
+            .get("layout.flexbox.enabled")
+            .as_bool()
+            .unwrap_or(false)
+    } else {
+        true
+    }
+}
+
 /// Defines an element’s display type, which consists of
 /// the two basic qualities of how an element generates boxes
 /// <https://drafts.csswg.org/css-display/#propdef-display>
 #[allow(missing_docs)]
 #[derive(Clone, Copy, Debug, Eq, FromPrimitive, Hash, MallocSizeOf, PartialEq, ToCss, ToShmem)]
 #[repr(u8)]
 pub enum DisplayOutside {
     None = 0,
@@ -58,17 +69,16 @@ pub enum DisplayOutside {
 #[derive(Clone, Copy, Debug, Eq, FromPrimitive, Hash, MallocSizeOf, PartialEq, ToCss, ToShmem)]
 #[repr(u8)]
 pub enum DisplayInside {
     None = 0,
     #[cfg(any(feature = "servo-layout-2020", feature = "gecko"))]
     Contents,
     Flow,
     FlowRoot,
-    #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
     Flex,
     #[cfg(feature = "gecko")]
     Grid,
     #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
     Table,
     #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
     TableRowGroup,
     #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
@@ -141,19 +151,17 @@ impl Display {
     pub const None: Self = Self::new(DisplayOutside::None, DisplayInside::None);
     #[cfg(any(feature = "servo-layout-2020", feature = "gecko"))]
     pub const Contents: Self = Self::new(DisplayOutside::None, DisplayInside::Contents);
     pub const Inline: Self = Self::new(DisplayOutside::Inline, DisplayInside::Flow);
     pub const InlineBlock: Self = Self::new(DisplayOutside::Inline, DisplayInside::FlowRoot);
     pub const Block: Self = Self::new(DisplayOutside::Block, DisplayInside::Flow);
     #[cfg(feature = "gecko")]
     pub const FlowRoot: Self = Self::new(DisplayOutside::Block, DisplayInside::FlowRoot);
-    #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
     pub const Flex: Self = Self::new(DisplayOutside::Block, DisplayInside::Flex);
-    #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
     pub const InlineFlex: Self = Self::new(DisplayOutside::Inline, DisplayInside::Flex);
     #[cfg(feature = "gecko")]
     pub const Grid: Self = Self::new(DisplayOutside::Block, DisplayInside::Grid);
     #[cfg(feature = "gecko")]
     pub const InlineGrid: Self = Self::new(DisplayOutside::Inline, DisplayInside::Grid);
     #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
     pub const Table: Self = Self::new(DisplayOutside::Block, DisplayInside::Table);
     #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
@@ -312,30 +320,29 @@ impl Display {
         Display::Inline
     }
 
     /// <https://drafts.csswg.org/css2/visuren.html#x13>
     #[cfg(feature = "servo")]
     #[inline]
     pub fn is_atomic_inline_level(&self) -> bool {
         match *self {
-            Display::InlineBlock => true,
+            Display::InlineBlock | Display::InlineFlex => true,
             #[cfg(any(feature = "servo-layout-2013"))]
-            Display::InlineFlex | Display::InlineTable => true,
+            Display::InlineTable => true,
             _ => false,
         }
     }
 
     /// Returns whether this `display` value is the display of a flex or
     /// grid container.
     ///
     /// This is used to implement various style fixups.
     pub fn is_item_container(&self) -> bool {
         match self.inside() {
-            #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
             DisplayInside::Flex => true,
             #[cfg(feature = "gecko")]
             DisplayInside::Grid => true,
             _ => false,
         }
     }
 
     /// Returns whether an element with this display type is a line
@@ -427,22 +434,19 @@ impl ToCss for Display {
             Display::WebkitInlineBox => dest.write_str("-webkit-inline-box"),
             #[cfg(feature = "gecko")]
             Display::MozInlineBox => dest.write_str("-moz-inline-box"),
             #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
             Display::TableCaption => dest.write_str("table-caption"),
             _ => match (outside, inside) {
                 #[cfg(feature = "gecko")]
                 (DisplayOutside::Inline, DisplayInside::Grid) => dest.write_str("inline-grid"),
+                (DisplayOutside::Inline, DisplayInside::Flex) => dest.write_str("inline-flex"),
                 #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
-                (DisplayOutside::Inline, DisplayInside::Flex) |
-                (DisplayOutside::Inline, DisplayInside::Table) => {
-                    dest.write_str("inline-")?;
-                    inside.to_css(dest)
-                },
+                (DisplayOutside::Inline, DisplayInside::Table) => dest.write_str("inline-table"),
                 #[cfg(feature = "gecko")]
                 (DisplayOutside::Block, DisplayInside::Ruby) => dest.write_str("block ruby"),
                 (_, inside) => {
                     if self.is_list_item() {
                         if outside != DisplayOutside::Block {
                             outside.to_css(dest)?;
                             dest.write_str(" ")?;
                         }
@@ -462,22 +466,21 @@ impl ToCss for Display {
 
 /// <display-inside> = flow | flow-root | table | flex | grid | ruby
 /// https://drafts.csswg.org/css-display/#typedef-display-inside
 fn parse_display_inside<'i, 't>(
     input: &mut Parser<'i, 't>,
 ) -> Result<DisplayInside, ParseError<'i>> {
     Ok(try_match_ident_ignore_ascii_case! { input,
         "flow" => DisplayInside::Flow,
+        "flex" if flexbox_enabled() => DisplayInside::Flex,
         #[cfg(any(feature = "servo-layout-2020", feature = "gecko"))]
         "flow-root" => DisplayInside::FlowRoot,
         #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
         "table" => DisplayInside::Table,
-        #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
-        "flex" => DisplayInside::Flex,
         #[cfg(feature = "gecko")]
         "grid" => DisplayInside::Grid,
         #[cfg(feature = "gecko")]
         "ruby" => DisplayInside::Ruby,
     })
 }
 
 /// <display-outside> = block | inline | run-in
@@ -570,20 +573,18 @@ impl Parse for Display {
         // Now parse the single-keyword `display` values.
         Ok(try_match_ident_ignore_ascii_case! { input,
             "none" => Display::None,
             #[cfg(any(feature = "servo-layout-2020", feature = "gecko"))]
             "contents" => Display::Contents,
             "inline-block" => Display::InlineBlock,
             #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
             "inline-table" => Display::InlineTable,
-            #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
-            "-webkit-flex" => Display::Flex,
-            #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
-            "inline-flex" | "-webkit-inline-flex" => Display::InlineFlex,
+            "-webkit-flex" if flexbox_enabled() => Display::Flex,
+            "inline-flex" | "-webkit-inline-flex" if flexbox_enabled() => Display::InlineFlex,
             #[cfg(feature = "gecko")]
             "inline-grid" => Display::InlineGrid,
             #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
             "table-caption" => Display::TableCaption,
             #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
             "table-row-group" => Display::TableRowGroup,
             #[cfg(any(feature = "servo-layout-2013", feature = "gecko"))]
             "table-header-group" => Display::TableHeaderGroup,