HTML:

<!DOCTYPE html>
<html>
    <head lang="en">
        <meta charset="UTF-8">
        <title></title>
        <link type="text/css" rel="stylesheet" href="/index.css">
    </head>
    <body>
        <a href="#foo" id="link">Click Here</a>
    </body>
</html>

CSS:

#link {
     display : block;
     width: 400px;
     height: 400px;
     background: #BADA55;
     border-radius: 200px;
     text-align: center;
     line-height: 400px;
 }

Example.