jq如何判断css是否存在

<button id="btn">检测</button>

<script src="http://www.lingchenliang.com/download/jquery-1.12.0.min.js"></script>

<script>

$("#btn").click(function(){

    $.ajax({

        url: './main.css',

        type: 'GET',

        complete: function(response){

            if(response.status == 200){

                alert('有效');

            }else{

                alert('无效');

            }

        }

    });

});

</script>