fix: relax parent validation

This reverts #13255 / #13158, and helps with the validation error part of #14120

The revert is justified because people should be free to render a component in isolation and then append it somewhere where they know the rendered output is valid. If someone renders a tag at the top level that is invalid without a certain parent (which itself is an edge case), then the likelihood that this is done on purpose is much higher than that it's done by accident.

.changeset/cool-gifts-sniff.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: relax html parent validation

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

@@ -167,25 +167,23 @@ 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 | null} parent_tag
+ * @param {string} parent_tag
  * @returns {boolean}
  */
 export function is_tag_valid_with_parent(tag, parent_tag) {
 	if (tag.includes('-') || parent_tag?.includes('-')) return true; // custom elements can be anything
 
-	if (parent_tag !== null) {
-		const disallowed = disallowed_children[parent_tag];
+	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

@@ -34,14 +34,12 @@ function stringify(element) {
 
 /**
  * @param {Payload} payload
- * @param {Element | null} parent
+ * @param {Element} parent
  * @param {Element} child
  */
 function print_error(payload, parent, child) {
 	var message =
-		(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`) +
+		`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;
@@ -85,8 +83,6 @@ 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;

packages/svelte/tests/runtime-runes/samples/invalid-html-ssr/_config.js

@@ -5,7 +5,7 @@ export default test({
 		dev: true
 	},
 
-	html: `<p></p><h1>foo</h1><p></p><form></form> hello`,
+	html: `<p></p><h1>foo</h1><p></p><form></form>`,
 
 	recover: true,
 
@@ -13,8 +13,7 @@ export default test({
 
 	errors: [
 		'node_invalid_placement_ssr: `<p>` (main.svelte:6:0) cannot contain `<h1>` (h1.svelte:1:0)\n\nThis can cause content to shift around as the browser repairs the HTML, and will likely result in a `hydration_mismatch` warning.',
-		'node_invalid_placement_ssr: `<form>` (main.svelte:9:0) cannot contain `<form>` (form.svelte:1:0)\n\nThis can cause content to shift around as the browser repairs the HTML, and will likely result in a `hydration_mismatch` warning.',
-		'node_invalid_placement_ssr: `<td>` (main.svelte:12:0) needs a valid parent element\n\nThis can cause content to shift around as the browser repairs the HTML, and will likely result in a `hydration_mismatch` warning.'
+		'node_invalid_placement_ssr: `<form>` (main.svelte:9:0) cannot contain `<form>` (form.svelte:1:0)\n\nThis can cause content to shift around as the browser repairs the HTML, and will likely result in a `hydration_mismatch` warning.'
 	],
 
 	warnings: [

packages/svelte/tests/runtime-runes/samples/invalid-html-ssr/main.svelte

@@ -9,4 +9,3 @@
 <form>
 	<Form />
 </form>
-<td>hello</td>