Continuing from #1909 (comment).
Based on test expectation, the set parameter can be floats. But the result is expected to floor down to integer.

The question is:

1. Why should the result be integral? https://github.com/web-platform-tests/wpt/blob/7da2887747da3ae5215b88e22d61a0fb69e13acb/webdriver/tests/classic/set_window_rect/set.py#L41
2. Why do we round down? https://github.com/web-platform-tests/wpt/blob/7da2887747da3ae5215b88e22d61a0fb69e13acb/webdriver/tests/classic/set_window_rect/set.py#L160

It seems neither is specified in Spec.