diff --git a/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp b/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp
index 6727d9444b..e492ef57b1 100644
--- a/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp
+++ b/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp
@@ -571,20 +571,24 @@ void BlockFormattingContext::layout_initial_containing_block(LayoutMode layout_m
     icb.build_stacking_context_tree();
 
     icb.set_width(viewport_rect.width());
-
-    layout_block_level_children(root(), layout_mode);
+    icb.set_height(viewport_rect.height());
 
     VERIFY(!icb.children_are_inline());
+    layout_block_level_children(root(), layout_mode);
 
-    // FIXME: The ICB should have the height of the viewport.
-    //        Instead of auto-sizing the ICB, we should spill into overflow.
+    // Compute scrollable overflow.
     float lowest_bottom = 0;
     icb.for_each_child_of_type<Box>([&](auto& child) {
         lowest_bottom = max(lowest_bottom, child.absolute_rect().bottom());
     });
 
-    // FIXME: This is a hack and should be managed by an overflow mechanism.
-    icb.set_height(max(static_cast<float>(viewport_rect.height()), lowest_bottom));
+    if (lowest_bottom >= viewport_rect.height()) {
+        auto& overflow_data = icb.ensure_overflow_data();
+        overflow_data.scrollable_overflow_rect = viewport_rect.to_type<float>();
+        overflow_data.scrollable_overflow_rect.set_height(lowest_bottom);
+    } else {
+        icb.clear_overflow_data();
+    }
 }
 
 static Gfx::FloatRect rect_in_coordinate_space(const Box& box, const Box& context_box)
diff --git a/Userland/Libraries/LibWeb/Layout/Box.h b/Userland/Libraries/LibWeb/Layout/Box.h
index 1841af67c1..914157c42d 100644
--- a/Userland/Libraries/LibWeb/Layout/Box.h
+++ b/Userland/Libraries/LibWeb/Layout/Box.h
@@ -16,6 +16,11 @@ namespace Web::Layout {
 
 class Box : public NodeWithStyleAndBoxModelMetrics {
 public:
+    struct OverflowData {
+        Gfx::FloatRect scrollable_overflow_rect;
+        Gfx::FloatPoint scroll_offset;
+    };
+
     const Gfx::FloatRect absolute_rect() const;
 
     Gfx::FloatPoint effective_offset() const;
@@ -129,6 +134,24 @@ public:
     bool has_intrinsic_height() const { return intrinsic_height().has_value(); }
     bool has_intrinsic_aspect_ratio() const { return intrinsic_aspect_ratio().has_value(); }
 
+    bool has_overflow() const { return m_overflow_data; }
+
+    Optional<Gfx::FloatRect> scrollable_overflow_rect() const
+    {
+        if (!m_overflow_data)
+            return {};
+        return m_overflow_data->scrollable_overflow_rect;
+    }
+
+    OverflowData& ensure_overflow_data()
+    {
+        if (!m_overflow_data)
+            m_overflow_data = make<OverflowData>();
+        return *m_overflow_data;
+    }
+
+    void clear_overflow_data() { m_overflow_data = nullptr; }
+
 protected:
     Box(DOM::Document& document, DOM::Node* node, NonnullRefPtr<CSS::StyleProperties> style)
         : NodeWithStyleAndBoxModelMetrics(document, node, move(style))
@@ -152,6 +175,8 @@ private:
     WeakPtr<LineBoxFragment> m_containing_line_box_fragment;
 
     OwnPtr<StackingContext> m_stacking_context;
+
+    OwnPtr<OverflowData> m_overflow_data;
 };
 
 template<>
diff --git a/Userland/Services/WebContent/PageHost.cpp b/Userland/Services/WebContent/PageHost.cpp
index 0414a88044..31cccb5ac1 100644
--- a/Userland/Services/WebContent/PageHost.cpp
+++ b/Userland/Services/WebContent/PageHost.cpp
@@ -102,7 +102,11 @@ void PageHost::page_did_layout()
 {
     auto* layout_root = this->layout_root();
     VERIFY(layout_root);
-    auto content_size = enclosing_int_rect(layout_root->absolute_rect()).size();
+    Gfx::IntSize content_size;
+    if (layout_root->has_overflow())
+        content_size = enclosing_int_rect(layout_root->scrollable_overflow_rect().value()).size();
+    else
+        content_size = enclosing_int_rect(layout_root->absolute_rect()).size();
     m_client.async_did_layout(content_size);
 }