diff --git a/log/s016.dj b/log/s016.dj
new file mode 100644
index 0000000..defe807
--- /dev/null
+++ b/log/s016.dj
@@ -0,0 +1,38 @@
+# S016 - Web UI behavior tests
+
+One isolated place: `app0/web/tests/`. Vitest + happy-dom — no real
+browser.
+
+## Layout
+
+- `tests/server.ts` - **FakeBff**: the production `linji serve` API
+  contract in memory (invite gate, bare-name normalization, sessions,
+  member-only space ACL, push endpoints). EventSource listeners are fed
+  `update` events when writes land in their space, mirroring the
+  server's zds watch.
+- `tests/setup.ts` - environment wiring: `fetch` -> FakeBff,
+  EventSource mock fed by the fake, Notification / PushManager /
+  serviceWorker mocks, `resetEnv()` between tests.
+- `tests/app.test.tsx` - 19 tests over the rendered App: login (hint,
+  invite field visibility, wrong invite, wrong password, bare name),
+  spaces (empty state, invalid URI, join, non-member denial, thread
+  expansion), posting (disabled button, post + composer clear, new
+  thread), SSE live update, open-app notifications (hidden tab fires,
+  own posts and visible tab don't), the bell (permission + push
+  subscription registration), logout.
+
+## Run
+
+`cd app0/web && bun test` (alias for `bunx vitest run`). ~1.5s.
+
+## Gotchas found (test-side)
+
+- Vitest resolves solid-js's `node` export condition (server build,
+  renders nothing). `vitest.config.ts` aliases `solid-js` and
+  `solid-js/web` to their browser `dev.js` builds.
+- `<For>` keys by reference: a refresh replaces the whole thread
+  section (new JSON objects), so assertions must re-query elements
+  after a refresh.
+- Seed/mutation state in the fake must be reset between tests
+  (`resetEnv` restores `inviteCode` too — one leaked mutation hid the
+  invite field for every later test).
diff --git a/roadmap.dj b/roadmap.dj
index 9fc6152..8b2d8d0 100644
--- a/roadmap.dj
+++ b/roadmap.dj
@@ -102,6 +102,8 @@ would be what fails.
 - **S015** v - SSE updates (`/api/events`, server watches zds as the
  session user; catches CLI-bot writes) + open-app notifications;
  closed-app delivery = best-effort. [log](log/s015.dj)
+- **S016** v - Web UI behavior tests: Vitest + happy-dom in
+  `app0/web/tests/` (fake in-memory BFF), 19 tests. [log](log/s016.dj)
 
 ## Open questions