diff --git a/.changeset/forty-books-taste.md b/.changeset/forty-books-taste.md
new file mode 100644
index 0000000000..e2474c635f
--- /dev/null
+++ b/.changeset/forty-books-taste.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: guard against `customElements` being unavailable in browser extension contexts
diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index 6656532986..a2fffe8696 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/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'
 		) {