Runtime hygiene

A feature that works while the console is full of errors is not finished. The errors are telling you about the paths you did not test.

Two gates: the browser must be quiet, and the toolchain must pass. Neither may be satisfied by suppression.

Gate 1: the browser is quiet

Run the flow and watch for all five:

Signal	Why it is not cosmetic
Console errors	Something threw. There is a path where your UI is broken.
React warnings	Keys, invalid nesting, `act`, controlled-to-uncontrolled — each predicts a specific class of bug you have not hit yet.
Hydration mismatches	The server and client rendered different HTML. React discards and re-renders, and the state that was there is now gone.
Failed requests	4xx and 5xx, and anything CORS-blocked. A silent failure is still a failure.
Unhandled promise rejections	An async path with no error handling. It will surface as a blank screen for someone.