CSS-背景渐变的兼容写法

background-image: -moz-linear-gradient(top, rgba(0,0,0,0), rgba(0,0,0,0.5) 75%);
    background-image: -webkit-linear-gradient(top, rgba(0,0,0,0), rgba(0,0,0,0.5) 75%);
    background-image: -o-linear-gradient(top, rgba(0,0,0,0), rgba(0,0,0,0.5) 75%);
    background-image: -ms-linear-gradient(top, rgba(0,0,0,0), rgba(0,0,0,0.5) 75%);
    background-image: linear-gradient(to bottom, rgba(0,0,0,0), rgba(0,0,0,0.5) 75%);