1. Within a GitHub Action that runs whenever code is pushed because this way the tests are run consistenly every time code is pushed. If you were to test manually, it would still work, but it is more effort and could easily be forgotten.
2. No, would use unit tests for function outputs
3. Navigation mode analyzes the page as it loads from the beginning. It measures overall load performance, like how fast the page becomes usable. Snapshot mode analyzes the page only in its current state after it has already loaded. It is mainly used for accessibility checks.
4. element does not have a [lang] attribute

Minifying JavaScript files can reduce payload sizes and script parse time

Reduce unused JavaScript and defer loading scripts until they are required to decrease bytes consumed by network activity