<style>
div{
width: 200px;
height: 35px;
border: 2px solid gray;
/*父相*/
position: relative;
}
span{
/*子绝:相对于父元素进行绝对定位*/
position: absolute;
top: 3px;
right:15px;
/*定个宽高*/
width: 15px;
height: 15px;
/*只要下边和右边*/
border-bottom:2px solid gray;
border-right: 2px solid gray;
/*旋转形成下三角*/
transform:rotate(45deg);
}
</style>
<div><span></span></div>
效果: