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.

For me, the primary difference came to light early on when I started working with box-shadow. Here’s a simple triangle not unlike the one I made back then.

Let’s use this to break down the difference between the two.

Box Shadow

Add a box-shadow on that bad boy and this happens.

It’s annoying, but makes sense. CSS uses a box model, where the element’s edges are bound in the shape of a rectangle. Even in cases where the shape of the element does not appear to be a box, the box is still there and that is was box-shadow is applied to. This was my “ah-ha moment” when understanding the box in box-shadow.

CSS Filter Drop Shadow

CSS Filters are pretty awesome. Take a gander at all the possibilities for adding visual filters on elements and marvel at how CSS suddenly starts doing a lot of things we used to have to mockup in Photoshop.

Filters are not bound to the box model. That means the outline of our triangle is recognized and the transparency around it is ignored so that the intended shape receives the shadow.

Deciding Which Method to Use

The answer is totally up to you. The simple example of a triangle above might make it seem that filter: drop-shadow() is better, but it’s not a fair comparison of the benefits or even the possibilities of both methods. It’s merely an illustration of their different behaviors in a specific context.

Like most things in development, the answer of which method to use depends. Here’s a side-by-side comparison to help distinguish the two and when it might be best to choose one over the other.

	Box Shadow	Drop Shadow
Specification	CSS Backgrounds and Borders Module Level 3	Filter Effects Module Level 1
Browser Support	Great	Good
Supports Spread Radius	Yes, as an optional fourth value	No
Blur Radius	Calculation is based on a pixel length	Calculation is based on the stdDeviation attribute of the SVG filter
Supports inset shadows	Yes	No
Performance	Not hardware accelerated	Hardware accelerated in browsers that support it. It’s a heavy lift without it.

Wrapping Up

The difference between box-shadow and filter: drop-shadow() really boils down to the CSS box model. One sees it and the other disregards it. There are other differences that distinguish the two in terms of browser support, performance and such, but the way the two treat the box model is the key difference.

Update: Amelia identified another key difference in the comments where the spread of the radius for drop-shadow() is calculated differently than box-shadow and even that of text-shadow. That means that the spread radius you might specify in box-shadow is not one-to-one with the default spread value for drop-shadow, so the two are not equal replacements of one another in some cases.

Let’s cap this off with a few other great examples illustrating that. Lennart Schoors also has a nice write-up with practical examples using tooltips and icons that we previously called out.