diff --git a/Userland/Libraries/LibWeb/Layout/Node.cpp b/Userland/Libraries/LibWeb/Layout/Node.cpp
index efc1a802ff..4afc3bacea 100644
--- a/Userland/Libraries/LibWeb/Layout/Node.cpp
+++ b/Userland/Libraries/LibWeb/Layout/Node.cpp
@@ -179,6 +179,43 @@ NodeWithStyle::NodeWithStyle(DOM::Document& document, DOM::Node* node, CSS::Comp
     m_font = Gfx::FontDatabase::default_font();
 }
 
+void NodeWithStyle::did_insert_into_layout_tree(CSS::StyleProperties const& style)
+{
+    // https://drafts.csswg.org/css-sizing-3/#definite
+    auto is_definite_size = [&](CSS::PropertyID property_id, bool width) {
+        // A size that can be determined without performing layout; that is,
+        // a <length>,
+        // a measure of text (without consideration of line-wrapping),
+        // a size of the initial containing block,
+        // or a <percentage> or other formula (such as the “stretch-fit” sizing of non-replaced blocks [CSS2]) that is resolved solely against definite sizes.
+        auto maybe_value = style.property(property_id);
+
+        auto const* containing_block = this->containing_block();
+        auto containing_block_has_definite_size = containing_block && (width ? containing_block->m_has_definite_width : containing_block->m_has_definite_height);
+
+        if (!maybe_value.has_value() || maybe_value.value()->has_auto()) {
+            // NOTE: The width of a non-flex-item block is considered definite if it's auto and the containing block has definite width.
+            if (width && is_block_container() && parent() && !parent()->computed_values().display().is_flex_inside())
+                return containing_block_has_definite_size;
+            return false;
+        }
+
+        auto maybe_length_percentage = style.length_percentage(property_id);
+        if (!maybe_length_percentage.has_value())
+            return false;
+        auto length_percentage = maybe_length_percentage.release_value();
+        if (length_percentage.is_length())
+            return true;
+        if (length_percentage.is_percentage())
+            return containing_block_has_definite_size;
+        // FIXME: Determine if calc() value is definite.
+        return false;
+    };
+
+    m_has_definite_width = is_definite_size(CSS::PropertyID::Width, true);
+    m_has_definite_height = is_definite_size(CSS::PropertyID::Height, false);
+}
+
 void NodeWithStyle::apply_style(const CSS::StyleProperties& specified_style)
 {
     auto& computed_values = static_cast<CSS::MutableComputedValues&>(m_computed_values);
@@ -436,27 +473,6 @@ void NodeWithStyle::apply_style(const CSS::StyleProperties& specified_style)
     if (computed_values.opacity() == 0)
         m_visible = false;
 
-    auto is_definite_size = [&](CSS::PropertyID property_id, bool width) {
-        auto maybe_value = specified_style.property(property_id);
-        if (!maybe_value.has_value() || maybe_value.value()->has_auto())
-            return false;
-        auto maybe_length_percentage = specified_style.length_percentage(property_id);
-        if (!maybe_length_percentage.has_value())
-            return false;
-        auto length_percentage = maybe_length_percentage.release_value();
-        if (length_percentage.is_length())
-            return true;
-        if (length_percentage.is_percentage()) {
-            auto* containing_block = this->containing_block();
-            return containing_block && (width ? containing_block->m_has_definite_width : containing_block->m_has_definite_height);
-        }
-        // FIXME: Determine if calc() value is definite.
-        return false;
-    };
-
-    m_has_definite_width = is_definite_size(CSS::PropertyID::Width, true);
-    m_has_definite_height = is_definite_size(CSS::PropertyID::Height, false);
-
     if (auto maybe_length_percentage = specified_style.length_percentage(CSS::PropertyID::Width); maybe_length_percentage.has_value())
         computed_values.set_width(maybe_length_percentage.release_value());
     if (auto maybe_length_percentage = specified_style.length_percentage(CSS::PropertyID::MinWidth); maybe_length_percentage.has_value())
diff --git a/Userland/Libraries/LibWeb/Layout/Node.h b/Userland/Libraries/LibWeb/Layout/Node.h
index 35429c2d8b..f65f263142 100644
--- a/Userland/Libraries/LibWeb/Layout/Node.h
+++ b/Userland/Libraries/LibWeb/Layout/Node.h
@@ -164,6 +164,8 @@ public:
     void set_has_definite_height(bool b) { m_has_definite_height = b; }
     void set_has_definite_width(bool b) { m_has_definite_width = b; }
 
+    void did_insert_into_layout_tree(CSS::StyleProperties const&);
+
 protected:
     NodeWithStyle(DOM::Document&, DOM::Node*, NonnullRefPtr<CSS::StyleProperties>);
     NodeWithStyle(DOM::Document&, DOM::Node*, CSS::ComputedValues);
diff --git a/Userland/Libraries/LibWeb/Layout/TreeBuilder.cpp b/Userland/Libraries/LibWeb/Layout/TreeBuilder.cpp
index bb453a7e43..5da3fc93cd 100644
--- a/Userland/Libraries/LibWeb/Layout/TreeBuilder.cpp
+++ b/Userland/Libraries/LibWeb/Layout/TreeBuilder.cpp
@@ -96,18 +96,19 @@ void TreeBuilder::create_layout_tree(DOM::Node& dom_node, TreeBuilder::Context&
     auto& document = dom_node.document();
     auto& style_computer = document.style_computer();
     RefPtr<Layout::Node> layout_node;
+    RefPtr<CSS::StyleProperties> style;
 
     if (is<DOM::Element>(dom_node)) {
         auto& element = static_cast<DOM::Element&>(dom_node);
         element.clear_pseudo_element_nodes({});
-        auto style = style_computer.compute_style(element);
+        style = style_computer.compute_style(element);
         if (style->display().is_none())
             return;
         element.set_specified_css_values(style);
-        layout_node = element.create_layout_node(move(style));
+        layout_node = element.create_layout_node(*style);
     } else if (is<DOM::Document>(dom_node)) {
-        auto style = style_computer.create_document_style();
-        layout_node = adopt_ref(*new Layout::InitialContainingBlock(static_cast<DOM::Document&>(dom_node), move(style)));
+        style = style_computer.create_document_style();
+        layout_node = adopt_ref(*new Layout::InitialContainingBlock(static_cast<DOM::Document&>(dom_node), *style));
     } else if (is<DOM::Text>(dom_node)) {
         layout_node = adopt_ref(*new Layout::TextNode(document, static_cast<DOM::Text&>(dom_node)));
     } else if (is<DOM::ShadowRoot>(dom_node)) {
@@ -150,6 +151,9 @@ void TreeBuilder::create_layout_tree(DOM::Node& dom_node, TreeBuilder::Context&
         insert_node_into_inline_or_block_ancestor(layout_node);
     }
 
+    if (layout_node->has_style() && style)
+        static_cast<Layout::NodeWithStyle&>(*layout_node).did_insert_into_layout_tree(*style);
+
     auto* shadow_root = is<DOM::Element>(dom_node) ? verify_cast<DOM::Element>(dom_node).shadow_root() : nullptr;
 
     if ((dom_node.has_children() || shadow_root) && layout_node->can_have_children()) {