gradient 线性渐变 浏览器兼容
filter: progid:DXImageTransform.Microsoft.gradient(GradientType=0, startColorstr=white, endColorstr=silver);/*IE<8-9>*/
background-image:-webkit-linear-gradient(top,white,silver);
background-image:-moz-linear-gradient(top,white,silver);
background-image:-ms-linear-gradient(top,white,silver);
background-image:linear-gradient(top,white,silver);