支持三大浏览器的背景渐变

css：

.gradual{

width: 600px;

height: 30px;

background-image:-webkit-linear-gradient(left, rgba(255,255,255,.1) 0%, rgba(121, 121, 121, 1) 50%,rgba(255,255,255,.1) 100%);

background-image:-webkit-o-gradient(left, rgba(255,255,255,.1) 0%, rgba(121, 121, 121, 1) 50%,rgba(255,255,255,.1) 100%);

background-image:-moz-linear-gradient(left, rgba(255,255,255,.1) 0%, rgba(121, 121, 121, 1) 50%,rgba(255,255,255,.1) 100%);

}

html结构：

<div class="gradual"></div>