实现向内凹陷圆角效果在原来的容器元素中定义4个角方块让方块的背景颜色跟容器元素的父级元素的背景相同分别给每个方块定义

在原来的容器元素中定义4个角方块
让方块的背景颜色跟容器元素的父级元素的背景相同
分别给每个方块定义圆角，并指定每个方块的边框跟容器元素相同
判断清楚各个方向的圆角位置，以及显示的那条边框

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