?> Background：:point_right: box-shadow, outline

We know that box-shadow will stick to the border-radius rounded edges, but the stroke outline will not fit the rounded edge border-radius, we can combine them. Using box-shadow to fill the gap created by the stroke outline to perform the effect we want.

!>About setting the value of expansion radius? Assuming that the radius of the rounded border-radius is r, according to the Pythagorean theorem, the minimum value of the expansion radius should be equal to (√2−1)r ~= 3.314, and the maximum value cannot exceed the stroke width, which is 6px.