Selenium

The original browser automation framework that started it all. Supports Chrome, Firefox, Edge and Safari through WebDriver protocol. Massive community means every problem you'll hit already has a StackOverflow answer. The API is a bit clunky compared to modern alternatives and you'll spend some time fighting with waits and selectors, but it's battle-tested and works with basically every language: Python, Java, JavaScript, C#, Ruby. If you need something that just works and has been around forever, this is it.

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