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SCSS style guide

This style guide recommends best practices for SCSS to make styles easy to read, easy to maintain, and performant for the end-user.

Rules

Utility Classes

As part of the effort for cleaning up our CSS and moving our components into gitlab-ui led by the GitLab UI WG we prefer the use of utility classes over adding new CSS. However, complex CSS can be addressed by adding component classes.

Where are utility classes defined?

Where should I put new utility classes?

New utility classes should be added to utilities.scss. Existing classes include:

Name Pattern Example
Background color .bg-{variant}-{shade} .bg-warning-400
Text color .text-{variant}-{shade} .text-success-500
Font size .text-{size} .text-2
  • {variant} is one of 'primary', 'secondary', 'success', 'warning', 'error'
  • {shade} is one of the shades listed on colors
  • {size} is a number from 1-6 from our Type scale

When should I 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 unreusable classes. Also, the kind of classes that emerge from "utility-first" tend to be design-centered (e.g. .button, .alert, .card) rather than domain-centered (e.g. .security-report-widget, .commit-header-icon).

Examples of component classes that were created using "utility-first" include:

Inspiration:

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;
}

Formatting

You should always use a space before a brace, braces should be on the same line, each property should each get its own line, and there should be a space between the property and its value.

// Bad
.container-item {
  width: 100px; height: 100px;
  margin-top: 0;
}

// Bad
.container-item
{
  width: 100px;
  height: 100px;
  margin-top: 0;
}

// Bad
.container-item{
  width:100px;
  height:100px;
  margin-top:0;
}

// Good
.container-item {
  width: 100px;
  height: 100px;
  margin-top: 0;
}

Note that there is an exception for single-line rulesets, although these are not typically recommended.

p { margin: 0; padding: 0; }

Colors

HEX (hexadecimal) colors should use shorthand where possible, and should use lower case letters to differentiate between letters and numbers, e.g. #E3E3E3 vs. #e3e3e3.

// Bad
p {
  color: #ffffff;
}

// Bad
p {
  color: #FFFFFF;
}

// Good
p {
  color: #fff;
}

Indentation

Indentation should always use two spaces for each indentation level.

// Bad, four spaces
p {
    color: #f00;
}

// Good
p {
  color: #f00;
}

Semicolons

Always include semicolons after every property. When the stylesheets are minified, the semicolons will be removed automatically.

// Bad
.container-item {
  width: 100px;
  height: 100px
}

// Good
.container-item {
  width: 100px;
  height: 100px;
}

Shorthand

The shorthand form should be used for properties that support it.

// Bad
margin: 10px 15px 10px 15px;
padding: 10px 10px 10px 10px;

// Good
margin: 10px 15px;
padding: 10px;

Zero Units

Omit length units on zero values, they're unnecessary and not including them is slightly more performant.

// Bad
.item-with-padding {
  padding: 0px;
}

// Good
.item-with-padding {
  padding: 0;
}

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.

IDs

Don't use ID selectors in CSS.

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

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

Variables

Before adding a new variable for a color or a size, guarantee:

  • There isn't already one
  • There isn't a similar one we can use instead.

Linting

We use SCSS Lint to check for style guide conformity. It uses the ruleset in .scss-lint.yml, which is located in the home directory of the project.

To check if any warnings will be produced by your changes, you can run rake scss_lint in the GitLab directory. SCSS Lint will also run in GitLab CI 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 linters.

Fixing issues

If you want to automate changing a large portion of the codebase to conform to the SCSS style guide, you can use CSSComb. First install Node and NPM, then run npm install csscomb -g to install CSSComb globally (system-wide). Run it in the GitLab directory with csscomb app/assets/stylesheets to automatically fix issues with CSS/SCSS.

Note that this won't fix every problem, but it should fix a majority.

Ignoring issues

If you want a line or set of lines to be ignored by the linter, you can use // scss-lint:disable RuleName (more info):

// This lint rule is disabled because it is supported only in Chrome/Safari
// scss-lint:disable PropertySpelling
body {
  text-decoration-skip: ink;
}
// scss-lint:enable PropertySpelling

Make sure a comment is added on the line above the disable rule, otherwise the linter will throw a warning. DisableLinterReason is enabled to make sure the style guide isn't being ignored, and to communicate to others why the style guide is ignored in this instance.