That article covers a bug in implementations of blurring in browsers. It shows how when a 200px red circle is fully covered by a blur with radius 16px, today you will see a 232px sized blurred circle. But when the circle is shifted down from under the blur, so there is one pixel of space separating the two, you would still expect to see 15 px of blurred red circle. Instead it goes 18, 17, 16, 0, 0, for every successive frame. That’s a bug.