Explore effective testing strategies for functional user interfaces in this 19-minute conference talk from Lambda Days 2025. Examine the current limitations of frontend testing approaches, including the common reliance on slow and error-prone end-to-end tests and visual snapshots. Learn how functional programming principles can revolutionize UI testing by leveraging the properties of functional UI frameworks like Elm and Lustre to create faster, more reliable, and easier-to-maintain tests. Discover a comprehensive toolkit of testing approaches that can be implemented before resorting to heavy-duty tools like Cypress that can slow down continuous integration processes. Gain insights into what exactly needs to be tested in user interfaces and how functional programming paradigms can make the testing process more robust and efficient.