Better Modifier Classes with CSS Custom Properties

I’ve used Sass on many different sizes of projects with many different types of technologies. For the longest time it was a basic necessity—regular CSS just didn’t have powerful enough tools to manage larger or more complex styling tasks. Now, however, CSS has a trick up its sleeve that’s been pulling me away from Sass.

CSS Custom Properties are like Sass variables on steroids. Whereas Sass variables compile down to normal CSS properties as part of your build step, and are thus useless after the fact, CSS Custom Properties remain live on the page and require no build step. Chris Coyier has a more in-depth write-up on the differences between the two over at CSS-Tricks, so in this post I will just focus on a more targeted use case I encountered recently.


In this post:

The modifier class “problem”

While working on a particular concept for the redesign of my freelancing site, I wound up in the classic situation of having two components that, apart from a specific property, were otherwise identical. “A case for modifier classes,” I thought to myself. But writing a base class with modifier classes that override certain properties just feels…weird. I don’t think it’s wrong, per se, but it does feel a little silly having to write some CSS that you then immediately rewrite.

Let’s take a look at that old way of writing a typical button component with modifier classes, and then examine how we might use CSS Custom Properties to improve things.

“Old” modifier classes

In the past, you might’ve configured a “base” button style, then added modifier classes that override a specific set of properties, like their background-color or font-size:

See the Pen bshow-better-modifier-classes--old by Bobby Showalter (@bobbyshowalter) on CodePen.

Sass gives us the ability to DRY this up a little bit with lists, maps, and loops:

See the Pen bshow-better-modifier-classes--scss-old by Bobby Showalter (@bobbyshowalter) on CodePen.

In either case, we still have to explicitly type out all the properties we want to change. It feels clunky writing and rewriting background-size, etc. over and over again.

“New” modifiers

With CSS Custom Properties, we have more power at our disposal. Since the custom properties are part of CSS itself, it’s easier to see both what is intended to be modifiable, and make those changes without rewriting so many properties.

We can reassign the custom properties through inline styles (thus eliminating the modifier classes all together):

See the Pen bshow-better-modifier-classes--new-1 by Bobby Showalter (@bobbyshowalter) on CodePen.

Note: I realize the custom property names aren’t any shorter than the properties they’re meant to replace. In practice, I imagine you’d use any sort of naming convention that suits your fancy. :)

Or, if you prefer to use just classes, you can use your normal modifier classes to redefine only the custom properties. Because you’re using both the base and modifier classes together, the scope of your custom properties matches and the override works.

See the Pen bshow-better-modifier-classes--new-2 by Bobby Showalter (@bobbyshowalter) on CodePen.