I suppose rgba() would work here. After all, browser support for both box-shadow and rgba() is roughly the same.

/* 50% black box shadow */
box-shadow: 10px 10px 10px rgba(0, 0, 0, 0.5);

div {
    width: 200px;
    height: 50px;
    line-height: 50px;
    text-align: center;
    color: white;
    background-color: red;
    margin: 10px;
}

div.a {
  box-shadow: 10px 10px 10px #000;
}

div.b {
  box-shadow: 10px 10px 10px rgba(0, 0, 0, 0.5);
}

<div class="a">100% black shadow</div>
<div class="b">50% black shadow</div>

I've done graphic design for many years and this applies to anything you want to make a drop shadow for, it requires 4 layers but the effect is flawless in appearance.

Excellent tip for applying this to css.