【a】
<!DOCTYPE html>
<html>
<head>
<style type='text/css'>
a {
position: relative;
display: inline-block;
}
a span {
position: absolute;
top: 50%;
left: 50%;
margin-top: -1em;
margin-left: -50%;
width: 100%;
height: 2em;
color: #f90;
background-color: rgba(0, 0, 0, 0.5);
}
</style>
</head>
<body>
<a href="#"><img src="http://www.w3cschool.cn/style/download.png" /><span>Some
text</span></a>
</body>
</html>
