lbacks", "--foo, left, --bar");
  test_valid_value("position-try-fallbacks", "--foo, flip-start, left");
  test_valid_value("position-try-fallbacks", "--foo flip-start, left");
  test_valid_value("position-try-fallbacks", "left, --bar flip-start");

  test_invalid_value("position-try-fallbacks", "none, flip-start");
  test_invalid_value("position-try-fallbacks", "flip-block flip-block");
  test_invalid_value("position-try-fallbacks", "flip-block flip-inline flip-inline");
  test_invalid_value("position-try-