A mostly reasonable approach to CSS and Sass, based on Airbnb's Styleguide
A “rule declaration” is the name given to a selector (or a group of selectors) with an accompanying group of properties. Here's an example:
.listing {
font-size: 18px;
line-height: 1.2;
}
In a rule declaration, “selectors” are the bits that determine which elements in the DOM tree will be styled by the defined properties. Selectors can match HTML elements, as well as an element's class, ID, or any of its attributes. Here are some examples of selectors:
.my-element-class {
/* ... */
}
[aria-hidden] {
/* ... */
}
Finally, properties are what give the selected elements of a rule declaration their style. Properties are key-value pairs, and a rule declaration can contain one or more property declarations. Property declarations look like this:
/* some selector */ {
background: #f1f1f1;
color: #333;
}
- Use soft tabs (2 spaces) for indentation
- Prefer dashes over camelCasing in class names.
- Underscores are okay if you are using BEM (see ITCSS and BEM below).
- Do not use ID selectors
- When using multiple selectors in a rule declaration, give each selector its own line.
- Put a space before the opening brace
{
in rule declarations - In properties, put a space after, but not before, the
:
character. - Put closing braces
}
of rule declarations on a new line - Put blank lines between rule declarations
Bad
.avatar{
border-radius:50%;
border:2px solid white; }
.no, .nope, .not_good {
// ...
}
#lol-no {
// ...
}
Good
.avatar {
border-radius: 50%;
border: 2px solid white;
}
.one,
.selector,
.per-line {
// ...
}
- Prefer line comments (
//
in Sass-land) to block comments. - Prefer comments on their own line. Avoid end-of-line comments.
- Write detailed comments for code that isn't self-documenting:
- Uses of z-index
- Compatibility or browser-specific code
We encourage the combination of ITCSS and BEM for these reasons:
- It helps create clear, strict relationships between CSS and HTML
- It helps us create reusable, composable components
- It allows for less nesting and lower specificity
- It helps in building scalable stylesheets
ITCSS, or “Inverted Triangle CSS”, is an approach for writing CSS that divides your styles up into layers from least to most specific (hence “Inverted Triangle”). It incorporates the concept of “object-oriented CSS”, which encourages you to think of your styles as a collection of “objects”: reusable, repeatable snippets that can be used independently throughout a website. ITCSS further makes a distinction between “components” (more specific) and “objects” (more generic).
The layers are as follows:
- Settings, e.g.
_settings.colors.scss
,_settings.spacing.scss
- Tools, e.g.
_tools.media-queries.scss
,_tools.typography.scss
- Generic, e.g.
_generic.reset.scss
,_generic.fonts.scss
- Elements, e.g.
_elements.page.scss
,_elements.forms.scss
- Objects, e.g.
_objects.block.scss
,_objects.panel.scss
- Components, e.g.
_components.button.scss
,_components.markdown-content-block.scss
- Trumps/Alterations/Utils, e.g.
_alterations.alignment.scss
,_alterations.hacks.scss
For details, see Harry Roberts' introduction to ITCSS on Creative Bloq.
Related reading:
- Jonathan Snook's SMACSS
- Nicole Sullivan's OOCSS wiki
- Smashing Magazine's Introduction to OOCSS
BEM, or “Block-Element-Modifier”, is a naming convention for classes in HTML and CSS. It was originally developed by Yandex with large codebases and scalability in mind, and can serve as a solid set of guidelines for implementing ITCSS.
We recommend using namespaces with ITCSS and BEM, so that objects have a o-
prefix, components have a c-
prefix, and utils have a u-
prefix.
- CSS Tricks' BEM 101
- Harry Roberts' introduction to BEM
- Harry Roberts' More Transparent UI Code with Namespaces
Example
// MarkdownContentBlock.jsx
function MarkdownContentBlock() {
return (
<section class="o-block c-markdown-content-block">
<h1 class="c-markdown-content-block__title">Lorem Ipsum</h1>
<article class="c-markdown-content-block__content">
<p>Vestibulum id ligula porta felis euismod semper.</p>
</article>
</section>
);
}
/* _components.markdown-content-block.scss */
.c-markdown-content-block { }
.c-markdown-content-block--featured { }
.c-markdown-content-block__title { }
.c-markdown-content-block__content { }
.c-markdown-content-block
is the “block” and represents the higher-level component.c-markdown-content-block__title
is an “element” and represents a descendant of.c-markdown-content-block
that helps compose the block as a whole..c-markdown-content-block--featured
is a “modifier” and represents a different state or variation on the standard.c-markdown-content-block
.
While it is possible to select elements by ID in CSS, it should generally be considered an anti-pattern. ID selectors introduce an unnecessarily high level of specificity to your rule declarations, and they are not reusable.
For more on this subject, read CSS Wizardry's article on dealing with specificity.
Avoid binding to the same class in both your CSS and JavaScript. Conflating the two often leads to, at a minimum, time wasted during refactoring when a developer must cross-reference each class they are changing, and at its worst, developers being afraid to make changes for fear of breaking functionality.
We recommend creating JavaScript-specific classes to bind to, prefixed with .js-
:
<button class="c-btn c-btn--primary js-share">Share</button>
Use 0
instead of none
to specify that a style has no border. (Both are valid; we prefer choosing one of the two for consistency and 0
is shorter so wins.)
Bad
.foo {
border: none;
}
Good
.foo {
border: 0;
}
Prefer rems
to other sizing units. Borders may be sized in px
. Spacing of text may be sized in em
s.
- Use the
.scss
syntax, never the original.sass
syntax - Order your regular CSS and
@include
declarations logically (see below)
-
Property declarations
Properties should be roughly ordered according to the SMACSS recommendation of:
- Box
- Border
- Background
- Text
- Transforms/Animations
- Other
@include
declarations should be inserted at the appropriate point, e.g..c-feature { position: absolute; @include spacing('pv', '1/2'); border: 0; @include font-size('s'); transform: translateX(-50%) translateY(-50%); }
-
Media queries
Inserting media queries at the end makes it easier to understand how components change with screen size.
.c-feature { position: absolute; @include spacing('pv', '1/2'); border: 0; @include font-size('s'); transform: translateX(-50%) translateY(-50%); @include media-query('m') { // ... } }
-
Nested selectors
Nested selectors, if necessary, go last, and nothing goes after them. Add whitespace between your rule declarations and nested selectors, as well as between adjacent nested selectors. Apply the same guidelines as above to your nested selectors.
.c-feature { position: absolute; @include spacing('pv', '1/2'); border: 0; @include font-size('s'); transform: translateX(-50%) translateY(-50%); @include media-query('m') { // ... } &__icon { margin-right: 1rem; } }
Prefer dash-cased variable names (e.g. $my-variable
) over camelCased or snake_cased variable names.
Mixins should be used to DRY up your code, add clarity, or abstract complexity--in much the same way as well-named functions. Mixins that accept no arguments can be useful for this, but note that if you are not compressing your payload (e.g. gzip), this may contribute to unnecessary code duplication in the resulting styles.
@extend
should be avoided because it has unintuitive and potentially dangerous behavior, especially when used with nested selectors. Even extending top-level placeholder selectors can cause problems if the order of selectors ends up changing later (e.g. if they are in other files and the order the files are loaded shifts). Gzipping should handle most of the savings you would have gained by using @extend
, and you can DRY up your stylesheets nicely with mixins.
Do not nest selectors more than three levels deep!
.page-container {
.content {
.profile {
// STOP!
}
}
}
When selectors become this long, you're likely writing CSS that is:
- Strongly coupled to the HTML (fragile) —OR—
- Overly specific (powerful) —OR—
- Not reusable
Again: never nest ID selectors!
If you must use an ID selector in the first place (and you should really try not to), they should never be nested. If you find yourself doing this, you need to revisit your markup, or figure out why such strong specificity is needed. If you are writing well formed HTML and CSS, you should never need to do this.
We are experimenting with moving away from SCSS and towards CSS-in-JS but have not settled on a standard solution yet. In the meantime, see our notes from our experiments in the ACB Handbook Wiki.