Twitter uses continuous testing to ensure its website functions properly across various platforms and browsers. They have thousands of unit tests and nearly a hundred integration tests, which are automated to reduce the need for manual QA efforts. The company uses Webdriver framework with Selenium tests written in JavaScript and BrowserStack for remote testing environments. To mitigate challenges related to test flakiness, Twitter implemented device-level retries and developed a visual tool called Test Dashboard to display success/failure rates over time. This robust testing infrastructure allows developers to focus on writing new code instead of ensuring the tests pass.