<style type="text/css"> 
 
    body { 
        background: url(pattern.png); 
        font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; 
    } 
    p { 
        color: rgb(225, 225, 225); 
        background: rgba(156, 217, 107, 0.25) none; 
        width: 400px; 
        margin: 48px auto; 
        padding: 32px 40px; 
    } 
    p:hover { 
        background: rgba(156, 217, 107, 0.50) none; 
    } 
 
</style> 
 
<!--[if lte IE 8]> 
    <style type="text/css"> 
 
        p { 
            background: transparent url(green-25.png); 
        } 
        p:hover { 
            background: transparent url(green-50.png); 
        } 
 
    </style> 
<![endif]-->

http://925html.com/code/rgba-ie-fallback/

RGBA — IE Fallback

ie, CSS3

CSS