In CSS, some properties have shorthand. One property that takes separated values. Syntactic sugar, as they say, to make authoring easier. Take transition, which might look something like:

.element {
  transition: border 0.2s ease-in-out;
}

We could have written it like this:

.element {
  transition-property: border;
  transition-duration: 0.2s;
  transition-timing-function: ease-in-out;
}

Every “part” of the shorthand value has its own property it maps to. But that’s not true for everything. Take box-shadow:

.element {
  box-shadow: 0 0 10px #333;
}

That’s not shorthand for other properties. There is no box-shadow-color or box-shadow-offset.

That’s where Custom Properties come to save us!

We could set it up like this:

:root {
  --box-shadow-offset-x: 10px;
  --box-shadow-offset-y: 2px;
  --box-shadow-blur: 5px;
  --box-shadow-spread: 0;
  --box-shadow-color: #333;
}

.element {
  box-shadow:
    var(--box-shadow-offset-x)
    var(--box-shadow-offset-y)
    var(--box-shadow-blur)
    var(--box-shadow-spread)
    var(--box-shadow-color);
}

A bit verbose, perhaps, but gets the job done.

Now that we’ve done that, remember we get some uniquely cool things:

  1. We can change individual values with JavaScript. Like:
    document.documentElement.style.setProperty("--box-shadow-color", "green");
  2. Use the cascade, if we need to. If we set --box-shadow-color: blue on any selector more specific than the :root, we’ll override that color.

Fallbacks are possible too, in case the variable isn’t set at all:

.element {
  box-shadow:
    var(--box-shadow-offset-x, 0)
    var(--box-shadow-offset-y, 0)
    var(--box-shadow-blur, 5px)
    var(--box-shadow-spread, 0)
    var(--box-shadow-color, black);
}

How about transforms? They are fun because they take a space-separated list of values, so each of them could be a custom property:

:root {
  --transform_1: scale(2);
  --transform_2: rotate(10deg);
}

.element{
  transform: var(--transform_1) var(--transform_2);
}

What about elements that do have individual properties for their shorthand, but also offer comma-separated multiple values? Another great use-case:

:root {
  --bgImage: url(basic_map.svg);
  --image_1_position: 50px 20px;
  --image_2_position: bottom right;
}

.element {
  background: 
    var(--bgImage) no-repeat var(--image_1_position),
    var(--bgImage) no-repeat var(--image_2_position);
}

Or transitions?

:root {
  --transition_1_property: border;
  --transition_1_duration: 0.2s;
  --transition_1_timing_function: ease;
  
  --transition_2_property: background;
  --transition_2_duration: 1s;
  --transition_2_timing_function: ease-in-out;
}

.element {
  transition: 
    var(--transition_1_property) 
    var(--transition_1_duration) 
    var(--transition_1_timing_function),
    var(--transition_2_property) 
    var(--transition_2_duration) 
    var(--transition_2_timing_function),
}

Dan Wilson recently used this kind of thing with animations to show how it’s possible to pause individual animations!


Here’s browser support:

This browser support data is from Caniuse, which has more detail. A number indicates that browser supports the feature at that version and up.

Desktop

Chrome Firefox IE Edge Safari
49 31 No 16 9.1

Mobile / Tablet

Android Chrome Android Firefox Android iOS Safari
87 83 81 9.3

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