fix: guard against `customElements` being unavailable in browser extension contexts (#14933)

fixes #14739

.changeset/forty-books-taste.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: guard against `customElements` being unavailable in browser extension contexts

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -221,7 +221,10 @@ export function set_custom_element_data(node, prop, value) {
 			// Don't compute setters for custom elements while they aren't registered yet,
 			// because during their upgrade/instantiation they might add more setters.
 			// Instead, fall back to a simple "an object, then set as property" heuristic.
-			setters_cache.has(node.nodeName) || customElements.get(node.tagName.toLowerCase())
+			setters_cache.has(node.nodeName) ||
+			// customElements may not be available in browser extension contexts
+			!customElements ||
+			customElements.get(node.tagName.toLowerCase())
 				? get_setters(node).includes(prop)
 				: value && typeof value === 'object'
 		) {