火狐浏览器 <a>元素无法居中

<!DOCTYPE html>
2048
<!--<script src="js/main.js" type="text/javascript"></script>-->
<!--<script type="text/javascript" src="js/show.js"></script>-->
<!--<script src="js/suport2048.js" type="text/javascript"></script>-->
<style>
     **header {
        display: block;
        margin: 0,auto;
        /*width: 500px;*/
        text-align: center;
    }** 

    header h1{
        font-family: Arial;
        font-size: 60px;
        font-weight: bold;
    }

     **header #newgame{
        display: block;
        margin: 20px,auto;
        width: 100px;
        padding: 10px;
        background-color: coral;
        text-decoration: none;
        font-family: Arial;
        color: white;
        border-radius: 10px;
    }** 

    header #newgame:hover{
        background-color: gold;
    }

    header p{
        font-family: Arial;
        font-size: 25px;
        margin: 20px,auto;
    }
</style>
2048

NEW GAME!
Score:0