It's a fact of CSS life that the 'border' of any block level element gets factored into it's final box size for layout. That means that if you add a border on a hover to an element that didn't already have a border of that exact size, you will cause a layout shift. It's one of my pet peeves, for sure, when I see it in final designs. I find those little shifts, even 1 pixel, jarring and awkward. (q