Test observable behavior, not implementation details, so refactoring does not break tests. Isolate UI knowledge in page or component objects, keep one condition per test, and use descriptive names, accepting a little duplication where it helps readability. Push checks down to the lowest level that can catch them, and delete high-level tests that lower-level ones already cover. Kent C. Dodds' guiding rule is that the more a test resembles real usage, the more confidence it gives, which favors user-facing queries over brittle internals.

Sources: Fowler, Selenium docs, Kent C. Dodds