color text-decoration不生效？

同样的设置，上面的代码color及text-decoration不生效，只有单独设置.tb-promo a{color： text-decoration}才能实现效果

``` <style>
    * {
        padding: 0;
        margin: 0;
    }

    .tb-promo {
        position: relative;
        width: 520px;
        height: 280px;
        top: 100px;
        left: 50%;
        margin-left: -260px;
    }

    .tb-promo img {
        width: 520px;
        height: 280px;
    }

    .tb-promo a {
        color: #fff;
        text-decoration: none;
    }

    .prev,
    .next {
        position: absolute;
        top: 50%;
        width: 20px;
        height: 30px;
        margin-top: -15px;
        background-color: rgba(0, 0, 0, 0.3);
        text-align: center;
        line-height: 30px;
    }

    .prev {
        left: 0;
        border-radius: 0 15px 15px 0;
    }

    .next {
        right: 0;
        border-radius: 15px 0 0 15px;
    }
style>

```css
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        .tb-promo {
            position: relative;
            width: 520px;
            height: 280px;
            background-color: pink;
            margin: 100px auto;
        }

        .tb-promo img {
            width: 520px;
            height: 280px;
        }
        .prev,
        .next {
            position: absolute;
            top: 50%;
            margin-top: -15px;
            width: 20px;
            height: 30px;
            background: rgba(0, 0, 0, .3);
            text-align: center;
            line-height: 30px;
            color: #fff;
            text-decoration: none;
        }

        .prev {
            left: 0;
            border-top-right-radius: 15px;
            border-bottom-right-radius: 15px;
        }

        .next {
            right: 0;
            border-top-left-radius: 15px;
            border-bottom-left-radius: 15px;
        }
    style>

可以看下css参考手册中的 css Properties text-decoration-color