Breaking down CSS Box Shadow vs. Drop Shadow

Drop shadows. Web designers have loved them for a long time to the extent that we used to fake them with PNG images before CSS Level 3 formally introduced them to the spec as the box-shadow property. I still reach for drop shadows often in my work because they add a nice texture in some contexts, like working with largely flat designs.

Not too long after box-shadow was introduced, a working draft for CSS Filters surfaced and, with it, a method for drop-shadow() that looks a lot like box-shadow at first glance. However, the two are different and it's worth comparing those differences.


Slicing SVG 9 Ways

Paul Lewis animated a 3D Card Flip. Straightforward enough there, but this needed a shadow element as well:

The shadow may need to move around or fade, so we need a solution that we can animate with transforms & opacity, since they can be handed off to the GPU. It will need to be separate to the element it’s shadowing, since we will likely want to fade and move it without affecting the other element.

He tried box-shadow and filter: blur(), but both aren't performant enough.

He tried an SVG element with a blur filter, but SVG alone can't do 9-slice scaling (necessary to keep the corners from scaling awkwardly).

The winner was using an SVG image but applying it to an element via border-image, which essentially 9-slice scaling.