背景色渐变兼容最好的方法，兼容IE

background: -moz-linear-gradient(top, rgba(0,0,0,0) 0%, rgba(0,0,0,0.9) 100%); 
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(0,0,0,0) 0%), color-stop(100%,rgba(0,0,0,0.9))); 
background: -webkit-linear-gradient(top, rgba(0,0,0,0) 0%,rgba(0,0,0,0.9) 100%); 
background: -o-linear-gradient(top, rgba(0,0,0,0) 0%,rgba(0,0,0,0.9) 100%); 
background: -ms-linear-gradient(top, rgba(0,0,0,0) 0%,rgba(0,0,0,0.9) 100%); 
background: linear-gradient(to bottom, rgba(0,0,0,0) 0%,rgba(0,0,0,0.9) 100%); 
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#00000000', endColorstr='#E5000000',GradientType=0 ); 

GradientType  取值 0-垂直渐变 , 1水平渐变

线性渐变： linear
径向渐变： radial
圆形： circle
椭圆： ellipse