In https://groups.google.com/a/chromium.org/d/msg/blink-dev/3B0I0EDKfpk/leSOUkN0AQAJ tkent@ reports that "Multiple test files in WPT check existence of onbeforeprint and onafterprint attributes, however I think there are no tests for event dispatching.  It's difficult to test printing with interoperable ways."

tkent@, there is a window.print() method, but I assume the problem is that the dialog is modal and the test would never finish?

There's already WebDriver support for dismissing prompts in https://w3c.github.io/webdriver/webdriver-spec.html#user-prompts so I think that with the work that gsnedders (geoffers) is doing on WebDriver in web-platform-tests, this may soon be possible.

(The test would have to request the prompt to be dismissed before calling print(), of course, so that could give rise to some interest race conditions.)