support css selectors on the client

packages/svelte/src/internal/client/dom/blocks/svelte-portal.js

@@ -1,7 +1,7 @@
 /** @import { TemplateNode } from '#client' */
 import { HYDRATION_END, HYDRATION_START, HYDRATION_START_ELSE } from '../../../../constants.js';
 import { block, remove_nodes, render_effect } from '../../reactivity/effects.js';
-import { hydrate_node, hydrating, set_hydrate_node } from '../hydration.js';
+import { hydrate_node, hydrating, set_hydrate_node, set_hydrating } from '../hydration.js';
 import { get_next_sibling } from '../operations.js';
 
 const portals = new Map();
@@ -44,18 +44,29 @@ export function portal_outlet(node, id) {
  * @returns {void}
  */
 export function portal(target, content) {
+	const is_css_selector = typeof target === 'string';
 	const portal = portals.get(target);
-	if (!portal)
+
+	let previous_hydrating = false;
+	let previous_hydrate_node = null;
+	// TODO right now the portal is rendered before the given anchor. Is that confusing for people and they'd rather have it as
+	// the first child _inside_ the anchor if the anchor is an element?
+	var anchor = is_css_selector ? document.querySelector(target) : portal?.anchor;
+
+	if (!anchor)
 		throw new Error(
 			'TODO error code: No portal found for given target. Make sure portal target exists before referencing it'
 		);
 
-	let previous_hydrate_node = null;
-	var anchor = portal.anchor;
-
 	if (hydrating) {
-		previous_hydrate_node = hydrate_node;
+		previous_hydrating = true;
-		set_hydrate_node((anchor = /** @type {TemplateNode} */ (get_next_sibling(portal.anchor))));
+		if (is_css_selector) {
+			// These are not SSR'd, so temporarily disable hydration to properly insert them
+			set_hydrating(false);
+		} else {
+			previous_hydrate_node = hydrate_node;
+			set_hydrate_node((anchor = /** @type {TemplateNode} */ (get_next_sibling(portal.anchor))));
+		}
 	}
 
 	const effect = block(() => {
@@ -69,8 +80,12 @@ export function portal(target, content) {
 		};
 	});
 
-	if (hydrating) {
+	if (previous_hydrating) {
-		portal.anchor = hydrate_node; // so that next head block starts from the correct node
+		if (is_css_selector) {
-		set_hydrate_node(/** @type {TemplateNode} */ (previous_hydrate_node));
+			set_hydrating(true);
+		} else {
+			portal.anchor = hydrate_node; // so that next head block starts from the correct node
+			set_hydrate_node(/** @type {TemplateNode} */ (previous_hydrate_node));
+		}
 	}
 }

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

@@ -180,6 +180,10 @@ export function portal_outlet(payload, id) {
  * @returns {void}
  */
 export function portal(payload, target, content) {
+	payload.out += EMPTY_COMMENT;
+
+	if (typeof target === 'string') return; // targets a CSS selector - not possible in SSR, ignore
+
 	const portal = payload.portals.get(target);
 	if (!portal)
 		throw new Error(
@@ -191,7 +195,6 @@ export function portal(payload, target, content) {
 	content(tmp_payload);
 	tmp_payload.out += EMPTY_COMMENT;
 	portal.content.push(tmp_payload.out);
-	payload.out += EMPTY_COMMENT;
 }
 
 /**