I'd go ahead with cypress, both for e2e testing (whole app at once, component integration etc) and component testing (testing components in isolation)

Check out storybook -> https://storybook.js.org/

People generally think of storybook as a place where you can view components in different configurations but you can reuse the “stories” for testing (in a kind of write once test everywhere kind of way)

I haven't used lit for more than testing viability testing where I am, but I've had success setting up jest with some vanilla is web components and doing unit testing that way.

I love Playwright. I've had better results (less flakiness) and better DX than with cypress. +1 for Playwright for me. Coupled with a CI that runs the tests and merge / publish only if everythings fine !

Give Web Test Runner a try

• runs in the browser - test in the same place you run
• extremely fast
• used by popular web components libraries like lit