Your tests pass on your machine. You push the code. The build breaks in production. Why? Because "works on my machine" is the most dangerous phrase in software testing.
When you run tests locally, you have your browser versions, your OS, your screen resolution, your network speed, your test data. The moment someone else runs those same tests on a different machine -- different results. CI/CD solves this. Every test runs in the same environment. Every time. No surprises.
Picture this. A team of 5 QA engineers. Each runs tests locally before raising a PR. Dev A runs on Mac with Chrome 120. Dev B runs on Windows with Chrome 119. Dev C forgets to run tests entirely. Dev D runs only the tests for their module. The QA lead runs the full suite once a week. Bugs slip through every single sprint.
Playwright automatically detects the CI environment. When process.env.CI is true, it runs in headless mode by default. Most CI providers (GitHub Actions, Jenkins, GitLab) set this variable automatically.
Q: Why should automated tests run in CI/CD instead of just locally?
A: Three reasons. First, consistency -- CI runs tests in the same environment every time, eliminating "works on my machine" issues. Second, enforcement -- tests run automatically on every push/PR, so no one can skip them. Third, history -- CI keeps a record of every test run, making it easy to spot trends like increasing flakiness or degrading performance. Local runs are for development. CI runs are for gatekeeping.
Key Point: CI/CD turns your tests from a development aid into a quality gate. Tests that only run locally are suggestions. Tests that run in CI are requirements.
Key Point: CI/CD ensures tests run consistently, automatically, and cannot be skipped. Local tests are suggestions. CI tests are requirements.