SCSS style guide

Utility Classes

In order to reduce the generation of more CSS as our site grows, prefer the use of utility classes over adding new CSS. In complex cases, CSS can be addressed by adding component classes.

Where are CSS utility classes defined?

Utility classes are generated by Tailwind CSS. There are three ways to view Tailwind CSS classes:

What CSS utility classes are deprecated?

Classes in utilities.scss and common.scss are being deprecated. Classes in common.scss that use non-design-system values should be avoided. Use classes with conforming values instead.

Avoid Bootstrap’s Utility Classes.

note
While migrating Bootstrap’s Utility Classes to the GitLab UI utility classes, note both the classes for margin and padding differ. The size scale used at GitLab differs from the scale used in the Bootstrap library. For a Bootstrap padding or margin utility, you may need to double the size of the applied utility to achieve the same visual result (such as ml-1 becoming gl-ml-2).

Tailwind CSS

As of August 2024, we are using Tailwind CSS as our CSS utilities provider. This replaces the previous, custom-built solution. See the Tailwind CSS design document for motivation, proposal, and implementation details.

Tailwind CSS basics

Below are some Tailwind CSS basics and information about how it has been configured to use the Pajamas design system. For a more in-depth guide see the official Tailwind CSS documentation.

Prefix

We have configured Tailwind CSS to use a prefix so all utility classes are prefixed with gl-. When using responsive utilities or state modifiers the prefix goes after the colon.

Examples: gl-mt-5, lg:gl-mt-5.

Responsive CSS utility classes

Responsive CSS utility classes are prefixed with the breakpoint name, followed by the : character. The available breakpoints are configured in tailwind.defaults.js#L44

Example: lg:gl-mt-5

Hover, focus, and other state modifiers

State modifiers can be used to conditionally apply any Tailwind CSS class. Prefix the CSS utility class with the name of the modifier, followed by the : character.

Example: hover:gl-underline

!important modifier

You can use the important modifier by adding ! to the beginning of the CSS utility class. When using in conjunction with responsive utility classes or state modifiers the ! goes after the : character.

Examples: !gl-mt-5, lg:!gl-mt-5, hover:!gl-underline

Spacing and sizing CSS utility classes

Spacing and sizing CSS utility classes (e.g. margin, padding, width, height) use our spacing scale defined in src/tokens/build/tailwind/tokens.cjs. See https://gitlab-org.gitlab.io/frontend/tailwind-documentation/margin for available CSS utility classes.

Example: gl-mt-5 is margin-top: 1rem;

Color CSS utility classes

Color CSS utility classes (e.g. color and background-color) use colors defined in src/tokens/build/tailwind/tokens.cjs. See https://gitlab-org.gitlab.io/frontend/tailwind-documentation/text-color for available CSS utility classes.

Example: gl-text-red-500 is color: var(--red-500, #dd2b0e);

Building the Tailwind CSS bundle

When using Vite or Webpack with the GitLab Development Kit, Tailwind CSS watches for file changes to build detected utilities on the fly.

To build a fresh Tailwind CSS bundle, run yarn tailwindcss:build. This is the script that gets called internally when building production assets with bundle exec rake gitlab:assets:compile.

However the bundle gets built, the output is saved to app/assets/builds/tailwind.css.

Tailwind CSS autocomplete

Tailwind CSS autocomplete lists all available classes in your code editor.

VS Code
note
If you are having trouble with slow autocomplete you may need to increase the amount of memory the TS server is allowed to use.

Install the Tailwind CSS IntelliSense extension. For HAML and custom *-class prop support these are the recommended settings: 

{
  "tailwindCSS.experimental.classRegex": [
    ["class: [\"|']+([^\"|']*)[\"|']+", "([a-zA-Z0-9\\-:!/]+)"],
    ["(\\.[\\w\\-.]+)[\\n\\=\\{\\s]", "([\\w\\-]+)"],
    ["[a-z]+-class(?:es)?=\"([^'\"]*)\""]
  ],
  "tailwindCSS.emmetCompletions": true
}
RubyMine

Tailwind CSS autocomplete is enabled by default. For full HAML and custom *-class prop support these are the recommended updates to the default settings: 

{
  "includeLanguages": {
    "haml": "html"
  },
  "emmetCompletions": true,
  "experimental": {
    "classRegex": [
      ["class: [\"|']+([^\"|']*)[\"|']+", "([a-zA-Z0-9\\-:!/]+)"],
      ["(\\.[\\w\\-.]+)[\\n\\=\\{\\s]", "([\\w\\-]+)"],
      ["[a-z]+-class(?:es)?=\"([^'\"]*)\""]
    ]
  }
}

Where should you put new utility classes?

Utility classes are generated by Tailwind CSS which supports most CSS features. If there is something that is not available we should update tailwind.defaults.js in GitLab UI.

When should you create component classes?

We recommend a “utility-first” approach.

  1. Start with utility classes.
  2. If composing utility classes into a component class removes code duplication and encapsulates a clear responsibility, do it.

This encourages an organic growth of component classes and prevents the creation of one-off non-reusable classes. Also, the kind of classes that emerge from “utility-first” tend to be design-centered (for example, .button, .alert, .card) rather than domain-centered (for example, .security-report-widget, .commit-header-icon).

Inspiration:

Leveraging Tailwind CSS in HTML and in stylesheets

When writing component classes, it’s important to effectively integrate Tailwind CSS’s utility classes to maintain consistency with the design system and keeping the CSS bundles small.

Utility CSS Classes in HTML vs. in stylesheets:

By using the utility classes directly in the HTML, we can keep the CSS file size smaller and adhere to the utility-first philosophy. By avoiding to combine utility classes with custom styles in one components class unless absolutely necessary, we can prevent confusion and potential conflicts.

  • Reasons for the Preference:
    • Smaller CSS File Size: Utilizing utility classes directly can lead to more compact CSS files and promote a more consistent design system.
    • Clarity and Maintainability: When utility classes are used in HTML, it’s clearer how styles are applied, reducing the risk of conflicts and regressions.
  • Potential Issues with Combining Styles:
    • Conflicts: If utility classes and custom styles are combined in a single class, conflicts can arise, especially when the styles have interdependencies.
    • Regressions: It becomes less obvious how styles should resolve, leading to possible regressions or unexpected behavior.

By following these guidelines, we can create clean, maintainable stylesheets that leverage Tailwind CSS effectively.

1. Use utility classes directly in HTML (preferred approach)

For better maintainability and to adhere to the utility-first principle, add utility classes directly to the HTML element. A component class should primarily contain only the non-utility CSS styles. In the following example, you add the utility classes gl-fixed and gl-inset-x-0, instead of adding position: fixed; right: 0; left: 0; to the SCSS file: 

<!-- Bad -->
<div class="my-class"></div>

<style>
  .my-class {
    top: $header-height;
    min-height: $comparison-empty-state-height;
    position: fixed;
    left: 0px;
    right: 0px;
 }
</style>

<!-- Good -->
<div class="my-class gl-fixed gl-inset-x-0"></div>

<style>
  .my-class {
    top: $header-height;
    min-height: $comparison-empty-state-height;
  }
</style>

2. Apply utility classes in component classes (when necessary)

Sometime it might not feasible to use utility classes directly in HTML and you need to include them in our custom SCSS files. Then, you might want to inherit style definitions from the design system without needing to figure out the relevant properties or values. To simplify this process, you can use Tailwind CSS’s @apply directive to include utilities’ style definitions in your custom styles.

Using @apply is encouraged for applying CSS properties that depend on the design system (e.g. margin, padding). For CSS properties that are unit-less (e.g display: flex) it is okay to use CSS properties directly. 

// Bad
.my-class {
  margin-top: 0.5rem;
}

// Okay
.my-class {
  display: flex;
}

// Good
.my-class {
  @apply gl-mt-5 gl-flex;
}

The preferred way to use @apply is to combine multiple CSS classes in a single line or at most two, like in the example above. This approach keeps the CSS concise and easy to read: 

// Good
.my-class {
  @apply gl-mt-5 gl-flex gl-items-center;
}

Avoid splitting classes across multiple lines, as shown below. 

// Avoid
@apply gl-mt-5;
@apply gl-flex;
@apply gl-items-center;

The reason for this is that IDE extensions might only be able to detect conflicts when the CSS Classes are in one line: 

// ✅ Conflict detected: 'gl-bg-black' applies the same CSS properties as 'gl-bg-white'.(cssConflict)
@apply gl-bg-white gl-bg-black;

// ❌ No conflict detected
@apply gl-bg-white;
@apply gl-bg-black;

The exception to this rule is when working with !important. Since !important applies to the entire line, each class that requires it should be applied on its own line. For instance: 

@apply gl-flex gl-items-center;
@apply gl-mt-5 #{!important};

This ensures that !important applies only where intended without affecting other classes in the same line.

Responsive design

Our UI should work well on mobile and desktop. To accomplish this we use CSS media queries. In general we should take a mobile first approach to media queries. This means writing CSS for mobile, then using min-width media queries to override styles on desktop. A exception to this rule is setting the display mode on child components. For example when hiding GlButton on mobile we don’t want to override the display mode set by our component CSS so we should use a max-width media query such as max-lg:gl-hidden.

Tailwind CSS classes 

<!-- Bad -->
<div class="gl-mt-5 max-lg:gl-mt-3"></div>

<!-- Good -->
<div class="gl-mt-3 md:gl-mt-5"></div>

<!-- Bad -->
<div class="gl-mt-3 sm:max-lg:gl-mt-5"></div>

<!-- Good -->
<div class="gl-mt-3 sm:gl-mt-5 lg:gl-mt-3"></div>

<!-- Bad -->
<!-- Changing the display mode of child components can cause visual regressions. -->
<gl-button class="gl-hidden lg:gl-flex">Edit</gl-button>

<!-- Good -->
<gl-button class="max-lg:gl-hidden">Edit</gl-button>

Component classes 

// Bad
.class-name {
  @apply gl-mt-5 max-lg:gl-mt-3;
}

// Good
.class-name {
  @apply gl-mt-3 lg:gl-mt-5;
}

// Bad
.class-name {
  display: block;

  @include media-breakpoint-down(lg) {
    display: flex;
  }
}

// Good
.class-name {
  display: flex;

  @include media-breakpoint-up(lg) {
    display: block;
  }
}

Naming

Filenames should use snake_case.

CSS classes should use the lowercase-hyphenated format rather than snake_case or camelCase. 

// Bad
.class_name {
  color: #fff;
}

// Bad
.className {
  color: #fff;
}

// Good
.class-name {
  color: #fff;
}

Avoid making compound class names with SCSS & features. It makes searching for usages harder, and provides limited benefit. 

// Bad
.class {
  &-name {
    color: orange;
  }
}

// Good
.class-name {
  color: #fff;
}

Class names should be used instead of tag name selectors. Using tag name selectors is discouraged because they can affect unintended elements in the hierarchy. 

// Bad
ul {
  color: #fff;
}

// Good
.class-name {
  color: #fff;
}

// Best
// prefer an existing utility class over adding existing styles

Class names are also preferable to IDs. Rules that use IDs are not-reusable, as there can only be one affected element on the page. 

// Bad
#my-element {
  padding: 0;
}

// Good
.my-element {
  padding: 0;
}

Nesting

Avoid unnecessary nesting. The extra specificity of a wrapper component makes things harder to override. 

// Bad
.component-container {
  .component-header {
    /* ... */
  }

  .component-body {
    /* ... */
  }
}

// Good
.component-container {
  /* ... */
}

.component-header {
  /* ... */
}

.component-body {
  /* ... */
}

Selectors with a js- Prefix

Do not use any selector prefixed with js- for styling purposes. These selectors are intended for use only with JavaScript to allow for removal or renaming without breaking styling.

Selectors with Util CSS Classes

Do not use utility CSS classes as selectors in your stylesheets. These classes are likely to change, requiring updates to the selectors and making the implementation harder to maintain. Instead, use another existing CSS class or add a new custom CSS class for styling elements. This approach improves maintainability and reduces the risk of bugs. 

// ❌ Bad
.gl-mb-5 {
  /* ... */
}

// ✅ Good
.component-header {
  /* ... */
}

Selectors with ARIA attributes

Do not use any attribute selector with ARIA for styling purposes. These attributes and roles are intended for supporting assistive technology. The structure of the components annotated with ARIA might change and so its styling. We need to be able to move these roles and attributes to different elements, without breaking styling. 

// Bad
&[aria-expanded=false] &-header {
  border-bottom: 0;
}

// Good
&.is-collapsed &-header {
  border-bottom: 0;
}

Using extend at-rule

Usage of the extend at-rule is prohibited due to memory leaks and the rule doesn’t work as it should.

Linting

We use stylelint to check for style guide conformity. It uses the ruleset in .stylelintrc and rules from our SCSS configuration. .stylelintrc is located in the home directory of the project.

To check if any warnings are produced by your changes, run yarn lint:stylelint in the GitLab directory. Stylelint also runs in GitLab CI/CD to catch any warnings.

If the Rake task is throwing warnings you don’t understand, SCSS Lint’s documentation includes a full list of their rules.