<div class="box">动画选框</div>

.box{
  width:300px;
  height:100px;
  border:1px dotted #fff;
  border-image:url(data:image/gif;base64,R0lGODlhCAAIAJEAAP////8A/wAAAAAAACH/C05FVFNDQVBFMi4wAwEAAAAh+QQFCgABACwAAAAACAAIAAACDwwuGckJzR4UaEp423InFAAh+QQFCgABACwAAAAACAAIAAACD4wEGcuMvR6cC4hpaU0nFAAh+QQJCgABACwAAAAACAAIAAACEAwiYhvd56KKaErqYDwnrQIAOw==) 1 repeat;
}