The worst test suite I have worked with had eleven hundred tests and nobody looked at the results. Roughly thirty failed on any given night, always different ones. The team's process was to re-run until it went green.
A suite in that state is worse than having none. It costs money to maintain, it costs time on every build, and it provides no safety, because a real failure looks exactly like the noise.
Four properties keep a suite out of that state. Every technique in this track serves one of them.
Stable. The same code gives the same result every time. A test that passes 90% of the time is not 90% useful. It is a coin flip that teaches your team to ignore red. Stability comes almost entirely from waiting for conditions instead of durations.
Independent. Each test creates what it needs and does not care what ran before it. Test order should not matter, and tests should survive running in parallel. Independence comes from unique test data and arranging your own state.
Repeatable. The test works on your laptop, on your colleague's, and on the build server. It does not depend on data someone created by hand last March, on your timezone, or on being run at a particular time of day.
Meaningful. The test would fail if the feature broke. This is the one people skip, and it is the one that decides whether any of the other three matter. A stable, independent, repeatable test that checks nothing is a well-engineered waste of time.
There is a quick way to check the fourth, and it is the idea this whole site is built on. Break the feature on purpose and confirm your test goes red. Change the expected count. Comment out the app's validation. If the test still passes, it was never testing that.
You do not have to do this by hand here. Every problem on this site is graded exactly this way: we take your passing test, break the app in a specific way, and re-run it. If it still passes, we tell you which bug it missed. That is the fastest feedback on assertion quality you can get.
Two habits that keep a suite healthy once it is written.
Fix or delete flaky tests, never re-run them. The moment a team's answer to red is "run it again", the suite has stopped working. A quarantine list is fine for a week. A quarantine list that lives for a year means those tests are gone and nobody admitted it.
A test that has never failed is unproven. Not wrong, unproven. You do not know it works until you have seen it catch something.
Remember this: stable, independent, repeatable, meaningful. Miss the last one and the first three are decoration.
You now have the ideas under every framework. Pick one and start writing: Cypress, Playwright or Selenium. The syntax is the easy part from here.