Heck, an extreme is what I did for Carakan, Opera's last in-house JS VM, where we were running the testsuite of pretty much any notable JS library in 2010 in CI

Heck, an extreme is what I did for Carakan, Opera's last in-house JS VM, where we were running the testsuite of pretty much any notable JS library in 2010 in CI; that caught loads of subtle bugs. (That said, obviously harder to automate, given there's no standard for in-browser testing!) Far as I'm aware, every browser is running some version of the jQuery testsuite nowadays, though I doubt that's the only third-party library tested in such a way in each browser.