fw-12 — Broader Ideas

Test layers answer different questions

A synthetic click verifies handler logic. It does not verify the element is reachable, visible, unobstructed, or activatable — because hit testing was skipped and isTrusted is false.

"We have 90% coverage" tells you nothing about which question your tests answer. Designing a test strategy is deciding which layer answers which question, and this module gives you the mechanism to argue it rather than assert it.

Accessibility as an engineering signal

A role query that cannot find your button is telling you something true about real users, through a channel engineers actually read. That makes testing-library-style queries a genuine accessibility signal — with the honest caveat that passing them is not the same as being usable.

The accname precedence order is worth knowing cold: aria-labelledby beats visible text, so an element can display one thing and be announced as another, and no amount of visual review catches it.

Everything DevTools shows is scriptable

DevTools is a CDP client, not privileged internals. Therefore traces, coverage, heap snapshots, accessibility trees, and layer counts can all go into CI.

Most teams look at these manually once and never again. Wiring the important ones into a pipeline is the Principal-level move — it converts a one-off investigation into a regression guard.

Observability must not perturb

A render-count tracker that retains component instances leaks; a profiler adding 40% overhead is measuring itself. This is bi-12's lesson at application scale, and it is the failure mode of most home-grown instrumentation.

Next

bi-16 uses this instrumentation to make the capstone's eight-layer explanation measurable rather than narrated.