根据
the bootstrap document，添加aria-hidden =“true”表示辅助技术跳过模态的DOM元素，这解释了主模态div中存在aria-hidden = true。
在模态标题div中为模态关闭按钮添加aria-hidden = true的目的是什么？
<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
<div class="modal-dialog">
<div class="modal-content">
**<*div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-hidden="true">× </button>
<h4 class="modal-title" id="myModalLabel">Modal title</h4>
</div>***
<div class="modal-body">
...
</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
<button type="button" class="btn btn-primary">Save changes</button>
</div>
</div><!-- /.modal-content -->
</div><!-- /.modal-dialog -->
</div><!-- /.modal -->