Using CSS variables

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This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for the proper prefixes to use in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the spec changes.

CSS Variables are entities defined by authors, or users, of Web pages to contain specific values throughout a document. They are set using custom properties and are accessed using a specific functional notation var().

Problems to solve

When building large sites, authors have been facing a maintainability challenge. In such Web sites, the size of the CSS is quite large and a lot of information in it is written at a multiple places. For example, maintaining a coherent color scheme throughout a document implies reusing a few color values at numerous positions in the CSS files. Altering the scheme, whether it is a tweaking a single color or completely rewriting is therefore a complex task, needing precision as a single find and replace often isn't enough.

The situation is especially worse with frameworks where changing colors require editing the framework itself. Preprocessors like LESS or SASS are quite helpful in these situations, but increase the complexity of the creation system, by adding an extra processing step.

A second advantage of these variables is that the name itself contains semantic information, allowing CSS files to be easier to read and understand: main-text-color is easier to understand than the reuse of #00ff00 throughout the text, especially if this same color is also used in another context.

How CSS Variables can help

In imperative programming languages, like Java, C++ or even JavaScript, the state can be tracked through the notion of variables. Variables are symbolic names associated with a given value, that can vary with the time.

In a declarative language like CSS, time-changing values are not common and the concept of variables is pretty uncommon.

Nevertheless, CSS introduces the notion of cascading variables in order to help solve the maintainability challenge. It allows for symbolically refering to a value throughout the CSS tree.

What are CSS Variables

CSS Variables currently have two forms:

  • variables, which are an association between an identifier and a value that can be used in place of any regular values, using the var() functional notation: var(example-variable) returns the value of the example-variable value.
  • custom properties, which are special properties of the form --* where * represent the variable name. These are used to define the value of a given variable: --example-variable: 20px; is a CSS declaration, using the custom --* property to set the value of the CSS variable example-variable to 20px.
Note: The custom property prefix was var- in the earlier spec, but later changed to --. Firefox 31 and above follow the new spec. (bug 985838)

Custom properties are similar to regular properties: they are subject to the cascade and inherit their value from their parent if not redefined.

First steps with CSS Variables

Let's start with this simple CSS that colors elements of different classes with the same color:

.one {
  color: white;
  background-color: brown;
  margin: 10px;
  width: 50px;
  height: 50px;
  display: inline-block;
}

.two {
  color: white;
  background-color: black;
  margin: 10px;
  width: 150px;
  height: 70px;
  display: inline-block;
}
.three {
  color: white;
  background-color: brown;
  margin: 10px;
  width: 75px;
}
.four {
  color: white;
  background-color: brown;
  margin: 10px;
  width: 100px;
}

.five {
  background-color: brown;
}

applied to this html:

<div>
    <div class="one"></div>
    <div class="two">Text <span class="five">- more text</span></div>
    <input class="three">
    <textarea class="four">Lorem Ipsum</textarea>
</div>

leads to:


There is a lot of repetition in the CSS. The background color, is set to brown in several places. For some CSS declarations, it is possible to declare this higher in the cascade and let CSS inheritance solve this problem naturally. For non-trivial projects, this is not always possible. By declaring a variable on the ::root pseudo-element, a CSS author can halt some instances of repetition by using the variable.

:root {
  --main-bg-color: brown;
}

.one {
  color: white;
  background-color: var(--main-bg-color);
  margin: 10px;
  width: 50px;
  height: 50px;
  display: inline-block;
}

.two {
  color: white;
  background-color: black;
  margin: 10px;
  width: 150px;
  height: 70px;
  display: inline-block;
}
.three {
  color: white;
  background-color:  var(--main-bg-color);
  margin: 10px;
  width: 75px;
}
.four {
  color: white;
  background-color:  var(--main-bg-color);
  margin: 10px;
  width: 100px;
}

.five {
  background-color:  var(--main-bg-color);
}

<div>
    <div class="one"></div>
    <div class="two">Text <span class="five">- more text</span></div>
    <input class="three">
    <textarea class="four">Lorem Ipsum</textarea>
</div>

leads to the same result as the previous example, yet allows for one canonical declaration of the desired property..

Inheritance of CSS Variables

Custom properties do inherit. It means that if no values is set for a custom property on a given element, the value of its parent is reused:

<div class="one">
  <div class="two">
    <div class="three">
    </div>
    <div class="four">
    </div>
  <div>
</div>

with the following CSS:

.two { --test: 10px; }
.three { --test: 2em; }

In this case, the result of var(--test) are:

  • for the class="two" element: 10px
  • for the class="three" element: 2em
  • for the class="four" element: 10px (inherited from its parent)
  • for the class="one" element: invalid value, which is the default value of any custom properties.

Validity and values

The classical CSS concept of validity, tied to each property, is not very useful in regard to custom properties. When the values of the custom properties are parsed, the browser doesn't know where they will be used, so must therefore consider nearly all values as valid.

Unfortunately these valid values can be used, via the var() functional notation, in a context where they don't make sense. Properties and custom variables can lead to invalid CSS statements leading to the new concept of valid at computed time.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support ? 29 (29) ? ? ?
Feature Android Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support ? 29 (29) ? ? ?

 

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Contributors to this page: Sheppy, AlexPl, jfla, velvel53, HumanBlade, ethertank, Ekanan, Yoshino, teoli, watilde
Last updated by: watilde,