chore: improve ssr parent validation (#13158)

.changeset/strange-trains-destroy.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: improve ssr parent validation

packages/svelte/src/html-tree-validation.js

@@ -162,21 +162,23 @@ export function is_tag_valid_with_ancestor(tag, ancestors) {
  * Returns false if the tag is not allowed inside the parent tag such that it will result
  * in the browser repairing the HTML, which will likely result in an error during hydration.
  * @param {string} tag
- * @param {string} parent_tag
+ * @param {string | null} parent_tag
  * @returns {boolean}
  */
 export function is_tag_valid_with_parent(tag, parent_tag) {
-	const disallowed = disallowed_children[parent_tag];
+	if (parent_tag !== null) {
+		const disallowed = disallowed_children[parent_tag];
 
-	if (disallowed) {
-		if ('direct' in disallowed && disallowed.direct.includes(tag)) {
-			return false;
-		}
-		if ('descendant' in disallowed && disallowed.descendant.includes(tag)) {
-			return false;
-		}
-		if ('only' in disallowed && disallowed.only) {
-			return disallowed.only.includes(tag);
+		if (disallowed) {
+			if ('direct' in disallowed && disallowed.direct.includes(tag)) {
+				return false;
+			}
+			if ('descendant' in disallowed && disallowed.descendant.includes(tag)) {
+				return false;
+			}
+			if ('only' in disallowed && disallowed.only) {
+				return disallowed.only.includes(tag);
+			}
 		}
 	}
 

packages/svelte/src/internal/server/dev.js

@@ -34,12 +34,14 @@ function stringify(element) {
 
 /**
  * @param {Payload} payload
- * @param {Element} parent
+ * @param {Element | null} parent
  * @param {Element} child
  */
 function print_error(payload, parent, child) {
 	var message =
-		`node_invalid_placement_ssr: ${stringify(parent)} cannot contain ${stringify(child)}\n\n` +
+		(parent === null
+			? `node_invalid_placement_ssr: ${stringify(child)} needs a valid parent element\n\n`
+			: `node_invalid_placement_ssr: ${stringify(parent)} cannot contain ${stringify(child)}\n\n`) +
 		'This can cause content to shift around as the browser repairs the HTML, and will likely result in a `hydration_mismatch` warning.';
 
 	if ((seen ??= new Set()).has(message)) return;
@@ -79,6 +81,8 @@ export function push_element(payload, tag, line, column) {
 			}
 			ancestor = ancestor.parent;
 		}
+	} else if (!is_tag_valid_with_parent(tag, null)) {
+		print_error(payload, null, child);
 	}
 
 	parent = child;