Only add HYDRATED_CSS when cmpTags are set

src/runtime/bootstrap-lazy.ts

@@ -185,7 +185,7 @@ export const bootstrapLazy = (lazyBundles: d.LazyBundlesRuntimeData, options: d.
     });
   });
 
-  if (BUILD.invisiblePrehydration && (BUILD.hydratedClass || BUILD.hydratedAttribute)) {
+  if (cmpTags.length > 0 && BUILD.invisiblePrehydration && (BUILD.hydratedClass || BUILD.hydratedAttribute)) {
     visibilityStyle.innerHTML = cmpTags + HYDRATED_CSS;
     visibilityStyle.setAttribute('data-styles', '');
 

src/runtime/test/bootstrap-lazy.spec.tsx

@@ -0,0 +1,30 @@
+import { doc } from '@platform';
+
+import { bootstrapLazy } from '../bootstrap-lazy';
+
+describe('assets', () => {
+  it('should not append broken css', () => {
+    const spy = jest.spyOn(doc.head, 'insertBefore');
+
+    /**
+     * To make the test shorter I called bootstrapLazy without any bundles
+     * When a user call defineCustomElements multiple times it will prevent defining the same elements within
+     * the bootstrapLazy method. Although this works it does still add the broken css.
+     */
+    bootstrapLazy([]);
+
+    expect(spy).not.toHaveBeenCalledWith(
+      expect.objectContaining({
+        sheet: expect.objectContaining({
+          cssRules: [
+            expect.objectContaining({
+              // This html is not valid since it does not start with a selector for the visibility hidden block
+              cssText: '{visibility:hidden}.hydrated{visibility:inherit}',
+            }),
+          ],
+        }),
+      }),
+      null,
+    );
+  });
+});