Hint: Think about what each shadow technique's shadow shape is actually calculated from — the element's rectangular box boundaries, or its actual rendered, potentially irregular pixel content.

box-shadow calculates and renders its shadow shape based purely on the element's own rectangular box model, its width, height, and border-radius if rounded, completely independent of what's actually visually rendered inside that box, which means an element containing a transparent PNG image with an irregular, non-rectangular visible shape, like a star or an object silhouette, still produces a plain rectangular shadow that doesn't match the image's actual visible silhouette at all. filter's drop-shadow() function, by contrast, calculates its shadow based on the element's actual rendered alpha channel, its real visible pixel shape after accounting for any transparency, meaning a transparent PNG's shadow correctly follows that irregular silhouette's true outline rather than the underlying rectangular box's boundaries. This distinction, box-boundary-based versus actual-rendered-shape-based shadow calculation, is exactly why drop-shadow() is the appropriate, more visually accurate choice specifically for adding a shadow to non-rectangular content like a transparent-background image, while box-shadow remains perfectly appropriate, and generally more performant, for straightforward rectangular or rounded-rectangle elements.