CSS3 线性渐变（linear-gradient），半透明+渐变
不兼容IE浏览器，所以IE中要添加默认的background或background-color
1、开始于center（水平方向）和top（垂直方向）也就是Top → Bottom：
/* Firefox 3.6+ */
background-image:-moz-linear-gradient(top, #ace, #f96);
/* Safari 5.1+, Chrome 10+ */
background-image:-webkit-linear-gradient(top, #ace, #f96);
/* Opera 11.10+ */
background-image:-o-linear-gradient(top, #ace, #f96);
2、始于left（水平方向）和center（垂直方向）也是就Left → Right：
/* Firefox 3.6+ */
background-image: -moz-linear-gradient(left, #ace, #f96);
/* Safari 5.1+, Chrome 10+ */
background-image: -webkit-linear-gradient(left, #ace, #f96);
/* Opera 11.10+ */
background-image: -o-linear-gradient(left, #ace, #f96);
3、起始于left（水平方向）和top（垂直方向）:
background-image: -moz-linear-gradient(left top, #ace, #f96);
background-image: -webkit-linear-gradient(left top, #ace, #f96);
background-image: -o-linear-gradient(left top, #ace, #f96);
4、角度(Angle)：
.deg45 {
background-image: -moz-linear-gradient(45deg, #ace, #f96);
background-image: -webkit-linear-gradient(45deg, #ace, #f96);
background-image: -o-linear-gradient(45deg, #ace, #f96);
}5、变透明+渐变
div{
background-image:linear-gradient(
to bottom
rgba(0,0,0,.6) 0%,
rgba(0,0,0,.3) 50%,
rgba(0,0,0,0) 100%
);
background-image:-webkit-linear-gradient(
bottom,
rgba(0,0,0,.6) 0%,
rgba(0,0,0,.3) 50%,
rgba(0,0,0,0) 100%
);
}rgba(0,0,0,0) 50%
rgba(0,0,0,0) 50%
rgba(0,0,0,.6) 0%,
