position:relative日常应用的时候一般是设置给position:absolute;的父层的,
父层position:relative; 子层position:absolute;的话, 就是依照父层的边界进行定位的, 

不然position:absolute 会逐层向上寻找设置了position:relative的元素边界, 直到body元素..

第一种写法（连同CSS一起追加进去）

	var freeOne="";
        freeOne=$(".freePreviewOne").attr("data-url");
        if(freeOne==null){
            //没有免费视频
        }else{
            $("#coursePicture").append("<a class='hide-650 fade5' "+
                    "style='top:94px;left:150px;position:absolute;z-index:100; " +
                    " width: 180px;height: 60px;border: 2px solid white;" +
                    "display: block;color: white;text-decoration: none;" +
                    "letter-spacing: 1px;font-size: 16px;line-height: 20px;" +
                    "text-align:center;padding-top:18px;" +
                    "background-color: rgba(0, 0, 0, 0.44);" +
                    "-webkit-backface-visibility: hidden;-webkit-transition: all .3s ease-in-out;" +
                    "-moz-transition: all .3s ease-in-out;-ms-transition: all .3s ease-in-out;" +
                    "-o-transition: all .3s ease-in-out;" +
                    "border-radius: 10px;'"+
                   
                    "href=http://www.mamicode.com/'#modal' data-toggle='modal' "+>页面：
<div class="es-row-wrap container-gap course-cover">

    <div class="row row-5-7 course-cover-heading">
      <div id="coursePicture" class="col-sm-5 picture hidden-xs" style="position: relative;">
        <#if course.coverImage?has_content>
          <img src=http://www.mamicode.com/"${course.coverImage}" class="img-responsive" width="436px" height="245px" />>
第二种写法：
//在课程图片上放置按钮
        /*  <a class="button hide-650 fade5" href="#modal" data-toggle="modal" data-url="/course/hyjgz2np/lesson/preview?lectureId=hyjgz2np0.49618492345325650.04212287697009742"> <span class="contactus"><strong>试看</strong></span>  </a>
         */
        
        //$("#coursePicture").append("<a class='btn btn-sm btn-info' href='#' style='top:120px;left:180px;position:absolute;z-index:100;'>试看</a>");
        var freeOne="";
        freeOne=$(".freePreviewOne").attr("data-url");
        if(freeOne==null){
            //没有免费视频
        }else{
            $("#coursePicture").append("<a class='freePreviewPicture hide-650 fade5' "+
                    "href=http://www.mamicode.com/'#modal' data-toggle='modal' "+>页面：
<style>
.freePreviewPicture{
    top:94px;
	left:150px;
	position:absolute;
	z-index:100;
    width: 180px;
	height: 60px;
	border: 2px solid white;
	display: block;
	color: white;text-decoration: none;
    letter-spacing: 1px;font-size: 16px;
	line-height: 20px;
    text-align:center;padding-top:18px;
    background-color: rgba(0, 0, 0, 0.44);
    -webkit-backface-visibility: hidden;
	-webkit-transition: all .3s ease-in-out;
    -moz-transition: all .3s ease-in-out;
	-ms-transition: all .3s ease-in-out;
    -o-transition: all .3s ease-in-out;
    border-radius: 10px;
}
</style>
<div class="es-row-wrap container-gap course-cover">

    <div class="row row-5-7 course-cover-heading">
      <div id="coursePicture" class="col-sm-5 picture hidden-xs" style="position: relative;">
        <#if course.coverImage?has_content>
          <img src=http://www.mamicode.com/"${course.coverImage}" class="img-responsive" width="436px" height="245px" />>
运行效果图：


JQuery与CSS之图片上放置按钮