diff --git a/experiments/WebScene.NativeEngine.Probe/native/webscene_css_compound.h b/experiments/WebScene.NativeEngine.Probe/native/webscene_css_compound.h
index 9e4b78b0..b5dc40fd 100644
--- a/experiments/WebScene.NativeEngine.Probe/native/webscene_css_compound.h
+++ b/experiments/WebScene.NativeEngine.Probe/native/webscene_css_compound.h
@@ -339,6 +339,8 @@ inline bool compound_matches(const Host& host,const dom_node& node,
                 if (!hash || !css::target_matches(node,*hash)) return false;
             } else if (name == "checked") {
                 if (!css::checked_matches(node)) return false;
+            } else if (name == "default") {
+                if (!css::default_matches(document,node)) return false;
             } else if (name == "indeterminate") {
                 if (!css::indeterminate_matches(node)) return false;
             } else if (name == "in-range") {
diff --git a/experiments/WebScene.NativeEngine.Probe/native/webscene_css_invalidation.h b/experiments/WebScene.NativeEngine.Probe/native/webscene_css_invalidation.h
index bcd37dcb..88b6c779 100644
--- a/experiments/WebScene.NativeEngine.Probe/native/webscene_css_invalidation.h
+++ b/experiments/WebScene.NativeEngine.Probe/native/webscene_css_invalidation.h
@@ -108,6 +108,16 @@ inline std::vector<css_compound_dependencies> compile_invalidation_plan(
             } else if (pseudo.name == "checked") {
                 for (const auto* name : {"checked", "selected", "type"})
                     add(output.attributes[name], route);
+            } else if (pseudo.name == "default") {
+                // Authored defaults update the subject directly. Submit-button
+                // ownership/order changes can select a different control, so
+                // those rare mutations conservatively recascade the document.
+                add(output.attributes["checked"], route);
+                add(output.attributes["selected"], route);
+                for (const auto* name : {"type", "form", "id"})
+                    output.attributes[name].scope |= invalidation_fallback;
+                output.child_list_sensitive = true;
+                output.child_list.scope |= invalidation_fallback;
             } else if (pseudo.name == "indeterminate") {
                 for (const auto* name : {"$live-form-indeterminate", "checked", "type", "name"})
                     add(output.attributes[name], route);
diff --git a/experiments/WebScene.NativeEngine.Probe/native/webscene_css_matching.h b/experiments/WebScene.NativeEngine.Probe/native/webscene_css_matching.h
index f918e970..ebd17d70 100644
--- a/experiments/WebScene.NativeEngine.Probe/native/webscene_css_matching.h
+++ b/experiments/WebScene.NativeEngine.Probe/native/webscene_css_matching.h
@@ -263,6 +263,73 @@ inline bool direction_matches(const native_document& document,const dom_node& no
                 }
                 return true;
 }
+inline const dom_node* form_owner_for_selector(
+    const native_document& document,const dom_node& control) {
+    const auto explicit_owner=control.attributes.find("form");
+    if(explicit_owner!=control.attributes.end()) {
+        if(explicit_owner->second.empty()) return nullptr;
+        const dom_node* root=&control;
+        while(auto* parent=document.dom_parent(*root)) {
+            if(parent->tag=="iframe") break;
+            root=parent;
+        }
+        const auto find=[&](const auto& recurse,const dom_node& current)->const dom_node* {
+            if(current.tag=="form" && current.id_attribute==explicit_owner->second)
+                return &current;
+            for(const auto* child:current.children) {
+                if(child==nullptr || child->tag=="iframe") continue;
+                if(const auto* matched=recurse(recurse,*child)) return matched;
+            }
+            return nullptr;
+        };
+        return find(find,*root);
+    }
+    for(auto* ancestor=document.dom_parent(control);ancestor!=nullptr;
+        ancestor=document.dom_parent(*ancestor)) {
+        if(ancestor->tag=="iframe") break;
+        if(ancestor->tag=="form") return ancestor;
+    }
+    return nullptr;
+}
+
+inline bool is_submit_button_for_selector(const dom_node& node) {
+    const auto authored=node.attributes.find("type");
+    const auto type=authored==node.attributes.end()
+        ? std::string_view{}:std::string_view{authored->second};
+    if(node.tag=="button") return type.empty() || html_keyword_equals(type,"submit");
+    return node.tag=="input"
+        && (html_keyword_equals(type,"submit") || html_keyword_equals(type,"image"));
+}
+
+inline bool default_matches(const native_document& document,const dom_node& node) {
+    if(node.tag=="option") return node.attributes.contains("selected");
+    if(node.tag=="input") {
+        const auto type=node.attributes.find("type");
+        if(type!=node.attributes.end()
+            && (html_keyword_equals(type->second,"checkbox")
+                || html_keyword_equals(type->second,"radio")))
+            return node.attributes.contains("checked");
+    }
+    if(!is_submit_button_for_selector(node)) return false;
+    const auto* owner=form_owner_for_selector(document,node);
+    if(owner==nullptr) return false;
+    const dom_node* root=&node;
+    while(auto* parent=document.dom_parent(*root)) {
+        if(parent->tag=="iframe") break;
+        root=parent;
+    }
+    const auto first=[&](const auto& recurse,const dom_node& current)->const dom_node* {
+        if(is_submit_button_for_selector(current)
+            && form_owner_for_selector(document,current)==owner) return &current;
+        for(const auto* child:current.children) {
+            if(child==nullptr || child->tag=="iframe") continue;
+            if(const auto* matched=recurse(recurse,*child)) return matched;
+        }
+        return nullptr;
+    };
+    return first(first,*root)==&node;
+}
+
 inline bool checked_matches(const dom_node& node) {
     const auto type=node.attributes.find("type");
     const bool checkable=node.tag=="input" && type!=node.attributes.end() &&