CSS实现内凹圆角，从而实现圆角边框

1、代码

<!DOCTYPE html>
<html><head><style>.uu {position: relative;width: 400px;height: 300px;}img {width: 100%;height: 100%;z-index: 1;}.box_right_top {background-image: radial-gradient(circle at left bottom, transparent 50px, white 0);position: absolute;top: 0;right: 0;z-index: 2;width: 50px;height: 50px;}.box_left_top {background-image: radial-gradient(circle at right bottom, transparent 50px, white 0);position: absolute;top: 0;left: 0;z-index: 2;width: 50px;height: 50px;}.box_right_bottom {background-image: radial-gradient(circle at left top, transparent 50px, white 0);position: absolute;bottom: 0;right: 0;z-index: 2;width: 50px;height: 50px;}.box_left_bottom {background-image: radial-gradient(circle at right top, transparent 50px, white 0);position: absolute;bottom: 0;left: 0;z-index: 2;width: 50px;height: 50px;}</style>
</head><body><div class="uu"><img src="https://fuss10.elemecdn.com/e/5d/4a731a90594a4af544c0c25941171jpeg.jpeg" /><div class="box_right_top"></div><div class="box_right_bottom"></div><div class="box_left_top"></div><div class="box_left_bottom"></div></div>
</body></html>

2、原理

使用了radial-gradient属性

3、效果图