CSS coding guidelines

Our CSS is written in Sass, using the SCSS syntax.


The SCSS source files are compiled to CSS using the gulp build system. This requires node.js to run. To install the libraries required for compiling the SCSS, run the following from the Wagtail repository root:

$ npm install

To compile the assets, run:

$ npm run build

Alternatively, the SCSS files can be monitored, automatically recompiling when any changes are observed, by running:

$ npm start

Linting SCSS

Wagtail uses the “scss-lint” Ruby Gem for linting.

Install it thus:

$ gem install scss-lint

Then run the linter from the wagtail project root:

$ scss-lint

The linter is configured to check your code for adherance to the guidelines below, plus a little more.


  • Use soft-tabs with a four space indent. Spaces are the only way to guarantee code renders the same in any person’s environment.
  • Put spaces after : in property declarations.
  • Put spaces before { in rule declarations.
  • Put line breaks between rulesets.
  • When grouping selectors, put each selector on its own line.
  • Place closing braces of declaration blocks on a new line.
  • Each declaration should appear on its own line for more accurate error reporting.
  • Add a newline at the end of your .scss files.
  • Strip trailing whitespace from your rules.
  • Add a space after the comma, in comma-delimited property values e.g rgba()


  • Use hex color codes #000 unless using rgba() in raw CSS (SCSS’ rgba() function is overloaded to accept hex colors as a param, e.g., rgba(#000, .5)).
  • Use // for comment blocks (instead of /* */).
  • Use single quotes for string values background: url('my/image.png') or content: 'moose'
  • Avoid specifying units for zero values, e.g., margin: 0; instead of margin: 0px;.
  • Strive to limit use of shorthand declarations to instances where you must explicitly set all the available values.

Sass imports

Leave off underscores and file extensions in includes:

// Bad
@import 'components/_widget.scss'

// Better
@import 'components/widget'

Pixels vs. ems

Use rems for font-size, because they offer absolute control over text. Additionally, unit-less line-height is preferred because it does not inherit a percentage value of its parent element, but instead is based on a multiplier of the font-size.

Specificity (classes vs. ids)

Always use classes instead of IDs in CSS code. IDs are overly specific and lead to duplication of CSS.

When styling a component, start with an element + class namespace, prefer direct descendant selectors by default, and use as little specificity as possible. Here is a good example:

<ul class="category-list">
    <li class="item">Category 1</li>
    <li class="item">Category 2</li>
    <li class="item">Category 3</li>
.category-list { // element + class namespace

    // Direct descendant selector > for list items
    > li {
        list-style-type: disc;

    // Minimal specificity for all links
    a {
        color: #f00;

Class naming conventions

Never reference js- prefixed class names from CSS files. js- are used exclusively from JS files.

Use the SMACSS is- prefix for state rules that are shared between CSS and JS.


As a rule of thumb, avoid unnecessary nesting in SCSS. At most, aim for three levels. If you cannot help it, step back and rethink your overall strategy (either the specificity needed, or the layout of the nesting).


Here are some good examples that apply the above guidelines:

// Example of good basic formatting practices
.styleguide-format {
    color: #000;
    background-color: rgba(0, 0, 0, .5);
    border: 1px solid #0f0;

// Example of individual selectors getting their own lines (for error reporting)
.get-new-lines {
    display: block;

// Avoid unnecessary shorthand declarations
.not-so-good {
    margin: 0 0 20px;
.good {
    margin-bottom: 20px;