.5)`);
    // FIXME: Clarify with spec editors if 'none' should pass through to the constants.
    test_valid_value(`color`, `hsl(from hsl(none none none) h s l)`,                            `rgb(0, 0, 0)`);
    test_valid_value(`color`, `hsl(from hsl(none none none / none) h s l / alpha)`,             `rgba(0, 0, 0, 0)`);
    test_valid_value(`color`, `hsl(from hsl(120deg none 50% / .5) h s l)`,                      `rgba(128, 128, 128, 0.5)`);
    test_valid_value(`color`, `hsl(from hsl(120deg 20% 50% / n