alpha)`, `rgba(51, 153, 255, 0.8)`);
    test_valid_value(`color`, `hwb(from rgb(20%, 40%, 60%, 80%) h w b / 0)`, `rgba(51, 102, 153, 0)`);

    // Testing replacement with a constant.
    test_valid_value(`color`, `hwb(from rebeccapurple 25 w b / alpha)`, `rgb(153, 94, 51)`);
    test_valid_value(`color`, `hwb(from rebeccapurple 25deg w b / alpha)`, `rgb(153, 94, 51)`);
    test_valid_value(`color`, `hwb(from rebeccapurple h 20% b / alpha)`, `rgb(102, 51, 153)`);
    test_valid_value(`color`, `hwb(from reb