rgba的含义，r代表red，g代表green，b代表blue，a代表透明度。红绿蓝是三原色，所有颜色都可以由这三种颜色拼合而成。比如 rgba(0,0,0,.5)就是透明度为0.5的黑色。现代浏览器是支持rgba的，但是在IE8等古董级浏览器中是不支持rgba的，IE8只能勉强 支持rgb()函数（即去掉了透明度，只能表示颜色）。
background: rgb(0, 0, 0); /*不支持rgba的浏览器*/
background: rgba(0,0,0,.5); /*支持rgba的浏览器*/
filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#7f000000,endColorstr=#7f000000); /*IE8支持*/
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原文地址:https://segmentfault.com/a/1190000002485299
感谢此博主的分享