diff --git a/components/layout/construct.rs b/components/layout/construct.rs
index 2e05b1ed694..53ddc2bed04 100644
--- a/components/layout/construct.rs
+++ b/components/layout/construct.rs
@@ -18,7 +18,7 @@ use context::LayoutContext;
 use data::{HAS_NEWLY_CONSTRUCTED_FLOW, PrivateLayoutData};
 use flex::FlexFlow;
 use floats::FloatKind;
-use flow::{MutableFlowUtils, MutableOwnedFlowUtils};
+use flow::{MutableFlowUtils, MutableOwnedFlowUtils, CAN_BE_FRAGMENTED};
 use flow::{self, AbsoluteDescendants, Flow, IS_ABSOLUTELY_POSITIONED, ImmutableFlowUtils};
 use flow_ref::{self, FlowRef};
 use fragment::{CanvasFragmentInfo, ImageFragmentInfo, InlineAbsoluteFragmentInfo};
@@ -1318,7 +1318,7 @@ impl<'a, 'ln, ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode<'ln>>
             return false
         }
 
-        if node.in_fragmentation_container() {
+        if node.can_be_fragmented() || node.style().is_multicol() {
             return false
         }
 
@@ -1616,7 +1616,13 @@ impl<'ln, ConcreteThreadSafeLayoutNode> NodeUtils for ConcreteThreadSafeLayoutNo
     }
 
     #[inline(always)]
-    fn set_flow_construction_result(self, result: ConstructionResult) {
+    fn set_flow_construction_result(self, mut result: ConstructionResult) {
+        if self.can_be_fragmented() {
+            if let ConstructionResult::Flow(ref mut flow, _) = result {
+                flow::mut_base(flow_ref::deref_mut(flow)).flags.insert(CAN_BE_FRAGMENTED);
+            }
+        }
+
         let mut layout_data = self.mutate_layout_data().unwrap();
         let dst = self.construction_result_mut(&mut *layout_data);
 
diff --git a/components/layout/flow.rs b/components/layout/flow.rs
index 2df03d43233..4a8b206193a 100644
--- a/components/layout/flow.rs
+++ b/components/layout/flow.rs
@@ -201,6 +201,27 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
         panic!("assign_block_size not yet implemented")
     }
 
+    /// Like `assign_block_size`, but is recurses explicitly into descendants.
+    /// Fit as much content as possible within `available_block_size`.
+    /// If that’s not all of it,
+    /// return an indication of where in the tree to break and start the next fragment.
+    ///
+    /// FIXME: replace `()` in the return value with something meaningful.
+    ///
+    /// The default is to make a flow "atomic": it can not be fragmented.
+    fn fragment<'a>(&mut self, ctx: &'a LayoutContext<'a>, _available_block_size: Au)
+                    -> Option<()> {
+        fn recursive_assign_block_size<'a, F: ?Sized + Flow>
+                                      (flow: &mut F, ctx: &'a LayoutContext<'a>) {
+            for child in mut_base(flow).children.iter_mut() {
+                recursive_assign_block_size(child, ctx)
+            }
+            flow.assign_block_size(ctx);
+        }
+        recursive_assign_block_size(self, ctx);
+        None
+    }
+
     /// If this is a float, places it. The default implementation does nothing.
     fn place_float_if_applicable<'a>(&mut self, _: &'a LayoutContext<'a>) {}
 
@@ -614,6 +635,9 @@ bitflags! {
                  static` and `position: relative` as well as absolutely-positioned flows with \
                  unconstrained positions in the block direction."]
         const BLOCK_POSITION_IS_STATIC = 0b0100_0000_0000_0000_0000,
+
+        /// Whether any ancestor is a fragmentation container
+        const CAN_BE_FRAGMENTED = 0b1000_0000_0000_0000_0000,
     }
 }
 
diff --git a/components/layout/multicol.rs b/components/layout/multicol.rs
index 3db1479dc52..f0c4acd09ea 100644
--- a/components/layout/multicol.rs
+++ b/components/layout/multicol.rs
@@ -11,7 +11,7 @@ use block::BlockFlow;
 use context::LayoutContext;
 use euclid::{Point2D, Rect};
 use floats::FloatKind;
-use flow::{Flow, FlowClass, OpaqueFlow};
+use flow::{Flow, FlowClass, OpaqueFlow, mut_base};
 use fragment::{Fragment, FragmentBorderBoxIterator};
 use std::fmt;
 use std::sync::Arc;
@@ -60,6 +60,10 @@ impl Flow for MulticolFlow {
 
     fn assign_block_size<'a>(&mut self, ctx: &'a LayoutContext<'a>) {
         debug!("assign_block_size: assigning block_size for multicol");
+        let available_block_size = Au(0);
+        for child in mut_base(self).children.iter_mut() {
+            child.fragment(ctx, available_block_size);
+        }
         self.block_flow.assign_block_size(ctx);
     }
 
diff --git a/components/layout/traversal.rs b/components/layout/traversal.rs
index b5f7505d834..69e85c39303 100644
--- a/components/layout/traversal.rs
+++ b/components/layout/traversal.rs
@@ -10,7 +10,7 @@
 use construct::FlowConstructor;
 use context::{LayoutContext, SharedLayoutContext};
 use flow::{PostorderFlowTraversal, PreorderFlowTraversal};
-use flow::{self, Flow};
+use flow::{self, Flow, CAN_BE_FRAGMENTED};
 use gfx::display_list::OpaqueNode;
 use incremental::{BUBBLE_ISIZES, REFLOW, REFLOW_OUT_OF_FLOW, REPAINT, RestyleDamage};
 use std::mem;
@@ -194,7 +194,10 @@ impl<'a> PostorderFlowTraversal for AssignBSizesAndStoreOverflow<'a> {
 
     #[inline]
     fn should_process(&self, flow: &mut Flow) -> bool {
-        flow::base(flow).restyle_damage.intersects(REFLOW_OUT_OF_FLOW | REFLOW)
+        let base = flow::base(flow);
+        base.restyle_damage.intersects(REFLOW_OUT_OF_FLOW | REFLOW)
+        // The fragmentation countainer is responsible for calling Flow::fragment recursively
+        && !base.flags.contains(CAN_BE_FRAGMENTED)
     }
 }
 
diff --git a/components/layout/wrapper.rs b/components/layout/wrapper.rs
index 652dd8c6359..cf02c66ed63 100644
--- a/components/layout/wrapper.rs
+++ b/components/layout/wrapper.rs
@@ -49,8 +49,8 @@ use script::dom::htmliframeelement::HTMLIFrameElement;
 use script::dom::htmlimageelement::LayoutHTMLImageElementHelpers;
 use script::dom::htmlinputelement::{HTMLInputElement, LayoutHTMLInputElementHelpers};
 use script::dom::htmltextareaelement::{HTMLTextAreaElement, LayoutHTMLTextAreaElementHelpers};
-use script::dom::node::{HAS_CHANGED, HAS_DIRTY_DESCENDANTS, IS_DIRTY};
-use script::dom::node::{IN_FRAGMENTATION_CONTAINER, LayoutNodeHelpers, Node, OpaqueStyleAndLayoutData};
+use script::dom::node::{CAN_BE_FRAGMENTED, HAS_CHANGED, HAS_DIRTY_DESCENDANTS, IS_DIRTY};
+use script::dom::node::{LayoutNodeHelpers, Node, OpaqueStyleAndLayoutData};
 use script::dom::text::Text;
 use script::layout_interface::TrustedNodeAddress;
 use selectors::matching::DeclarationBlock;
@@ -226,12 +226,12 @@ impl<'ln> TNode<'ln> for ServoLayoutNode<'ln> {
         self.node.set_flag(HAS_DIRTY_DESCENDANTS, value)
     }
 
-    fn in_fragmentation_container(&self) -> bool {
-        unsafe { self.node.get_flag(IN_FRAGMENTATION_CONTAINER) }
+    fn can_be_fragmented(&self) -> bool {
+        unsafe { self.node.get_flag(CAN_BE_FRAGMENTED) }
     }
 
-    unsafe fn set_in_fragmentation_container(&self, value: bool) {
-        self.node.set_flag(IN_FRAGMENTATION_CONTAINER, value)
+    unsafe fn set_can_be_fragmented(&self, value: bool) {
+        self.node.set_flag(CAN_BE_FRAGMENTED, value)
     }
 
     unsafe fn borrow_data_unchecked(&self) -> Option<*const PrivateStyleData> {
@@ -761,7 +761,7 @@ pub trait ThreadSafeLayoutNode<'ln> : Clone + Copy + Sized {
         }
     }
 
-    fn in_fragmentation_container(&self) -> bool;
+    fn can_be_fragmented(&self) -> bool;
 
     /// If this is a text node, generated content, or a form element, copies out
     /// its content. Otherwise, panics.
@@ -939,8 +939,8 @@ impl<'ln> ThreadSafeLayoutNode<'ln> for ServoThreadSafeLayoutNode<'ln> {
         }
     }
 
-    fn in_fragmentation_container(&self) -> bool {
-        self.node.in_fragmentation_container()
+    fn can_be_fragmented(&self) -> bool {
+        self.node.can_be_fragmented()
     }
 
     fn text_content(&self) -> TextContent {
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index 3f8bcefeef8..33dadfe527b 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -146,7 +146,7 @@ bitflags! {
         const SEQUENTIALLY_FOCUSABLE = 0x20,
 
         /// Whether any ancestor is a fragmentation container
-        const IN_FRAGMENTATION_CONTAINER = 0x40
+        const CAN_BE_FRAGMENTED = 0x40
     }
 }
 
diff --git a/components/style/dom.rs b/components/style/dom.rs
index 63c645901b3..ce538f1b21f 100644
--- a/components/style/dom.rs
+++ b/components/style/dom.rs
@@ -129,9 +129,9 @@ pub trait TNode<'ln> : Sized + Copy + Clone {
         }
     }
 
-    fn in_fragmentation_container(&self) -> bool;
+    fn can_be_fragmented(&self) -> bool;
 
-    unsafe fn set_in_fragmentation_container(&self, value: bool);
+    unsafe fn set_can_be_fragmented(&self, value: bool);
 
     /// Borrows the PrivateStyleData without checks.
     #[inline(always)]
diff --git a/components/style/matching.rs b/components/style/matching.rs
index f2a1515ef07..ea25d518a99 100644
--- a/components/style/matching.rs
+++ b/components/style/matching.rs
@@ -701,10 +701,10 @@ pub trait MatchMethods<'ln> : TNode<'ln> {
             // scope first.
             self.set_restyle_damage(damage);
 
-            self.set_in_fragmentation_container(
-                parent.as_ref().map_or(false, |p| p.in_fragmentation_container()) ||
-                self.borrow_data().unwrap().style.as_ref().unwrap().is_multicol()
-            );
+            self.set_can_be_fragmented(parent.map_or(false, |p| {
+                p.can_be_fragmented() ||
+                parent_style.as_ref().unwrap().is_multicol()
+            }));
         }
     }
 }
diff --git a/ports/geckolib/wrapper.rs b/ports/geckolib/wrapper.rs
index 77563f91ee3..a4d97c8a17c 100644
--- a/ports/geckolib/wrapper.rs
+++ b/ports/geckolib/wrapper.rs
@@ -182,13 +182,13 @@ impl<'ln> TNode<'ln> for GeckoNode<'ln> {
         unimplemented!()
     }
 
-    fn in_fragmentation_container(&self) -> bool {
+    fn can_be_fragmented(&self) -> bool {
         // FIXME(SimonSapin): Servo uses this to implement CSS multicol / fragmentation
         // Maybe this isn’t useful for Gecko?
         false
     }
 
-    unsafe fn set_in_fragmentation_container(&self, _value: bool) {
+    unsafe fn set_can_be_fragmented(&self, _value: bool) {
         // FIXME(SimonSapin): Servo uses this to implement CSS multicol / fragmentation
         // Maybe this isn’t useful for Gecko?
     }