Selenium

Selenium is the browser automation tool that started it all: clicking buttons, filling forms, scraping pages, running tests. It drives real browsers (Chrome, Firefox, Safari, Edge) programmatically, and it works with practically every programming language: Java, Python, C#, Ruby, JavaScript.

Selenium WebDriver talks directly to browser engines through standardized protocols. Write a test in Python, it opens Chrome, navigates to your app, clicks through your checkout flow, and tells you if something broke. The WebDriver protocol it pioneered is now a W3C standard that every browser implements.

Selenium Grid lets you run tests in parallel across multiple browsers and machines. For CI pipelines testing against Chrome, Firefox, and Safari simultaneously, Grid is the standard approach.

The catch: Selenium feels its age. Setup is fiddly: you need browser drivers that match your browser versions, and version mismatches cause cryptic errors. Tests are slower than modern alternatives because they drive full browsers through an HTTP protocol. Flaky tests are a known pain point. Playwright (from Microsoft) does everything Selenium does with better auto-waiting, built-in browser management, and faster execution. If you're starting fresh, Playwright or Cypress are better choices. Selenium's advantage is ecosystem size and language support: if you're in Java or C#, the alternatives are thinner.