rgba(128, 128, 128, 0.8)`);

    // Testing with calc().
    test_valid_value(`color`, `hwb(from rebeccapurple calc(h) calc(w) calc(b))`, `rgb(102, 51, 153)`);
    test_valid_value(`color`, `hwb(from rgb(20%, 40%, 60%, 80%) calc(h) calc(w) calc(b) / calc(alpha))`, `rgba(51, 102, 153, 0.8)`);

    // Testing with 'none'.
    test_valid_value(`color`, `hwb(from rebeccapurple none none none)`,                         `rgb(255, 0, 0)`);
    test_valid_value(`color`, `hwb(from rebeccapurple none none none / none