Welcome to the GitHub CSS Styleguide—it's pretty rad. Before continuing, you should have a general understanding for specificity, the SCSS syntax, and KSS documentation.

Coding style


  • Use soft-tabs with a two 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, keep individual selectors to a single line.
  • Place closing braces of declaration blocks on a new line.
  • Each declaration should appear on its own line for more accurate error reporting.


  • Use hex color codes #000 unless using rgba().
  • Use // for comment blocks (instead of /* */).
  • 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.


  • 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).
  • Document styles with KSS.


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;

File organization

In general, the CSS file organization should follow something like this:

├── components
│   ├── comments.scss
│   └── listings.scss
├── globals
│   ├── browser_helpers.scss
│   ├── responsive_helpers.scss
│   └── variables.scss
├── plugins
│   ├── jquery.fancybox-1.3.4.css
│   └── reset.scss
├── sections
│   ├── issues.scss
│   └── profile.scss
└── shared
    ├── forms.scss
    └── markdown.scss

Multiple bundles

Variations on this structure include the multi-bundle Rails approach, as seen in the github/github repo:

├── github
│   ├── base.scss
│   ├── layout.scss
│   └── buttons.scss
├── github2
│   ├── about.scss
│   └── blog.scss
└── mobile
    ├── _variables.scss
    └── base.scss

Here, we have two desktop bundles to support IE9's maximum selector limit per CSS file, as well as a dedicated mobile bundle to go with our separate mobile views.


Much of our core CSS comes from Primer, our internal (closed source) CSS library. It's a separate repository that we include via Bower to import common and global "GitHubby" styles like our Markdown type, grid system, buttons, and more.

Including (S)CSS files

Use Sprockets to require files. However, you should explicitly import any scss that does not generate styles (globals/) in the particular SCSS file you'll be needing it's helpers in. Here's a good example:

//= require_tree ./plugins
//= require my_awesome_styles

@import "../globals/basic";

.rule { ... }

This is also how Primer's styles are to be included, should you need them.

Pixels vs. ems

Use px for font-size, because it offers 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.

Class naming conventions

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

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

Specificity (classes vs. ids)

Elements that occur exactly once inside a page should use IDs, otherwise, use classes. When in doubt, use a class name.

  • Good candidates for ids: header, footer, modal popups.
  • Bad candidates for ids: navigation, item listings, item view pages (ex: issue view).

When styling a component, start with an element + class namespace (prefer class names over ids), 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;

CSS Specificity guidelines

  • If you must use an id selector (#selector) make sure that you have no more than one in your rule declaration. A rule like #header .search #quicksearch { ... } is considered harmful.
  • When modifying an existing element for a specific use, try to use specific class names. Instead of .listings-layout.bigger use rules like .listings-layout.listings-bigger. Think about ack/greping your code in the future.
  • The class names disabled, mousedown, danger, hover, selected, and active should always be namespaced by an element (button.selected is a good example).