diff --git a/crates/layout/src/lib.rs b/crates/layout/src/lib.rs
index abd1c9f..773ff5e 100644
--- a/crates/layout/src/lib.rs
+++ b/crates/layout/src/lib.rs
@@ -8462,6 +8462,76 @@ body { margin: 0; }
         );
     }
 
+    #[test]
+    fn form_with_block_children_stacks_vertically() {
+        // Regression: author CSS forcing `form > * { display: block }` must
+        // produce form controls that stack vertically (one per line) with
+        // their author margins honoured. Previously `<form>` defaulted to
+        // display:inline because the UA stylesheet was missing the `form`
+        // rule, which broke layout for block-display children.
+        let mut doc = Document::new();
+        let root = doc.root();
+        let html = doc.create_element("html");
+        let head = doc.create_element("head");
+        let style = doc.create_element("style");
+        let css = doc.create_text("form > * { display: block; margin: 8px 0; }");
+        let body = doc.create_element("body");
+        let form = doc.create_element("form");
+        let label1 = doc.create_element("label");
+        let label1_text = doc.create_text("Name");
+        let input1 = doc.create_element("input");
+        doc.set_attribute(input1, "type", "text");
+        let input2 = doc.create_element("input");
+        doc.set_attribute(input2, "type", "text");
+        doc.append_child(root, html);
+        doc.append_child(html, head);
+        doc.append_child(head, style);
+        doc.append_child(style, css);
+        doc.append_child(html, body);
+        doc.append_child(body, form);
+        doc.append_child(form, label1);
+        doc.append_child(label1, label1_text);
+        doc.append_child(form, input1);
+        doc.append_child(form, input2);
+
+        let tree = layout_doc(&doc);
+        let body_box = &tree.root.children[0];
+        let form_box = &body_box.children[0];
+
+        // form must lay out as a block (not be split as inline) and contain
+        // three stacked children.
+        assert!(
+            matches!(form_box.box_type, BoxType::Block(_)),
+            "<form> must be a block-level box (UA default); got {:?}",
+            form_box.box_type
+        );
+        assert!(
+            form_box.children.len() >= 3,
+            "form should contain its three element children, got {}",
+            form_box.children.len()
+        );
+
+        // Each in-flow child must have a positive height (no zero-sized
+        // collapse) and a distinct y-position from its predecessors.
+        let mut prev_bottom = f32::NEG_INFINITY;
+        for (i, child) in form_box.children.iter().enumerate() {
+            assert!(
+                child.rect.height > 0.0,
+                "form child {i} ({:?}) should have positive height, got {}",
+                child.box_type,
+                child.rect.height,
+            );
+            assert!(
+                child.rect.y >= prev_bottom,
+                "form child {i} ({:?}) at y={} should not overlap previous (bottom={})",
+                child.box_type,
+                child.rect.y,
+                prev_bottom,
+            );
+            prev_bottom = child.rect.y + child.rect.height;
+        }
+    }
+
     #[test]
     fn form_controls_are_atomic_inline() {
         // When multiple form controls are in a p, they should lay out side by side.
diff --git a/crates/style/src/computed.rs b/crates/style/src/computed.rs
index 6a10933..f9ec3d0 100644
--- a/crates/style/src/computed.rs
+++ b/crates/style/src/computed.rs
@@ -741,7 +741,7 @@ pub fn ua_stylesheet() -> Stylesheet {
 const UA_CSS: &str = r#"
 html, body, div, p, pre, h1, h2, h3, h4, h5, h6,
 ul, ol, li, blockquote, section, article, nav,
-header, footer, main, hr {
+header, footer, main, hr, form {
     display: block;
 }
 
@@ -3116,6 +3116,21 @@ mod tests {
         assert_eq!(div_node.style.display, Display::Block);
     }
 
+    #[test]
+    fn ua_form_is_block() {
+        // `<form>` defaults to display: block per HTML spec. Without this,
+        // author CSS like `form > * { display: block }` produces a
+        // block-in-inline situation that breaks layout for form controls.
+        let (mut doc, _, _, body) = make_doc_with_body();
+        let form = doc.create_element("form");
+        doc.append_child(body, form);
+
+        let styled = resolve_styles(&doc, &[], (800.0, 600.0)).unwrap();
+        let body_node = &styled.children[0];
+        let form_node = &body_node.children[0];
+        assert_eq!(form_node.style.display, Display::Block);
+    }
+
     #[test]
     fn ua_display_inline_elements() {
         let (mut doc, _, _, body) = make_doc_with_body();