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.