Introduction

All CSS in a codebase should be consistent and give the impression it was written by a sole developer. It should be understandable and simple to work with.

Syntax

Consistency is the most important factor when contributing to existing CSS. Generally the following structure should be used:

4 space indentation (no tabs)
1 declaration per line
A space after the colon separating the property and value pair
Lowercase strings
Always apply a semicolon to the last declaration
Each selector should have it’s own line
Include a space before the opening brace of a declaration block
Closing braces of a declaration block should be on a separate line
Quote attribute selectors with single quotes
Separate rules by new lines

// Do not format like this
.selector, .another-selector[attr=""]{
  border:0; line-height:inherit;
  width:auto}

// Format like this
.selector,
[attr=''] {
    border: 0; 
    line-height: inherit;
    width: auto;
}

###Strings Prefer single quotes over double quotes. Once a standard has been set, stick with it.

.selector {
  background-image: ('images/super-awesome-image.jpg');
  font-family: 'Helvetica';
}

###Declaration Order Alphabetical ordering is preferred. Although a very debatable subject, this style suits an unknown team size because of the following:

Consistency is the most important factor
Everyone knows the alphabet
Not all css contributors can distinguish between box-model properties, positioning, visual and typography
No learning curve required for custom ordering preferences

// Define declarations alphabetically
.selector {
    border: 1px solid rgba(0, 0, 0, .15);
    border-radius: 3px;
    box-shadow: inset 0px 1px 3px rgba(0, 0, 0, .1);
    color: #fff;
    width: auto;
}

###Units Values of 0 should be unitless. Favour rem over px. Favour seconds over milliseconds. Use relative units whenever possible such as on line-height. Avoid unnecessary leading zero’s.

// Do not do this
.selector {
    background-color: rgba(255, 255, 255, 0.4);
    border: 0px;
    font-size: 16px;
    line-height: 24px;
}

// Use relative units
.selector {
    background-color: rgba(255, 255, 255, .4);
    border: 0;
    font-size: 1rem;
    line-height: 1.5;
}

If fallback values are required use a mixin when using Sass, otherwise add a fallback in a browser specific file.

###Colours Colours should be written as lowercase hexademical values and in shorthand when possible. If transparency is required favour rgba. Sass can parse hex values for rgba so colour variables and map values can always be stored in hex.

// Do not do this
color: #FF6600;

// Do this
color: #f60;

###Shorthand Group properties into shorthand use when appropriate to avoid adding multiple properties:

// Avoid this
.selector {
    margin-bottom: 1rem;
    margin-left: 2rem;
    margin-right: 2rem;
    margin-top: 1rem;
}

// Do this instead
.selector {
    margin: 1rem 2rem;
}

Ensure that shorthand is not used repeatedly when it is not needed. If only one value needs changing do not use shorthand:

// Don't do this
body {
    font: normal 16px/1.5 'Helvetica';
}

.another-selector {
    font: normal 2rem/1.5 'Helvetica'; 
}

// Do this
body {
    font: normal 16px/1.5 'Helvetica';
}

.another-selector {
    font-size: 2rem; 
}

###Important rule Never use this. Rework selectors instead. You will reap the benefits in the long term.

###Vendor Prefixes Prefixes should never be used in Sass files and should be added via a build tool to ensure only required prefixes are added. IE specific vendor prefixes should be added to an IE only file or through the use of an IE mixin.

Selectors

###Naming Class and variable names should be lowercase and hyphen separated rather than camelcase or snakecase.

// Do not do this
.formList {}
.form_list {}

// Use hyphenated lowercase
.form-list {}

###ID’s Never use ID’s for selectors. If you are forced to reference an ID then use an attribute selector instead. This has the same specificity as a class and so can be overwritten without increasing specificity levels.

// Never use ID selectors
#anidselector {}

// Prefer an ID attribute if it is absolutely necessary
[id='anidselector'] {}

###Qualifying Never tag-qualify. By qualifying selectors specificity becomes a problem. Always rework your markup and class structure to address inheritance issues instead of qualifying.

// Instead of this
input[type='checkbox'] {}

// Use this
[type='checkbox'] {}

###JS-Classes Any classname prefixed with js- should not have any presentational properties attached. These are used as references for JS Classes. Instead is- prefixed classes should be used as state classes.

// Don't do this
.js-navigation {
    display: block;
}

// Use this
.is-active {
    &.navigation {
        display: block;
    }
}

This allows for the removal of JS without interfering with presentation.

Box Model

A document should use 1 box-model type consistently. Once a box-sizing value has been set there should not be any other elements using an alternate value.

// Do not do this
html,
body {
    box-sizing: border-box;
}

.element {
    box-sizing: content-box;
}

###Float Containment Clearing should be performed through a pseudo-class clearfix. Overflow clearing should not be used.

// Don't do this
.container {
    overflow: auto;
}

.child-element,
.child-element-2 {
    float: left;
    width: 50%;
}

// Do this
.container {
    &:after {
        clear: both;
        content: '';
        display: table;
    }
}

.child-element,
.child-element-2 {
    float: left;
    width: 50%;
}

Inheritance

Inheritance should be used whenever possible to avoid declaration duplications.

// Instead of duplicate declarations
.promotion-banner p,
.promotion-banner h1 {
    font-size: 1.2rem;
}

// Use inheritance
.promotion-banner {
    font-size: 1.2rem;
}

Avoid redeclaring values when they aren’t necessary:

// Don't do this
p {
    color: #666;
    margin: 1rem 0;
}

.parent {
    color: #000;
}

.parent p {
    color: #000;
    margin: 1rem 0 2rem;
}

// Use inheritance when possible
p {
    color: #666;
    margin: 1rem 0;
}

.parent {
    color: #000;
}

.parent p {
    color: inherit;
    margin-bottom: 2rem;
}

Comments

Comments should be added at the top of each component to indicate the purpose and use of the component. They should also be added above a selector when appropriate to explain references to other elements/classes/components. These should use the // syntax. e.g.

//
// If a form-horizontal parent modifier class is assigned, 
// the form-list component changes to a horizontal layout 
// for viewports larger than small screens
//
.form-horizontal & {
    @include breakpoint(map-get($breakpoints, 'small')) {
    }
}

Sass mixins and functions should also be marked with annotations providing a description, any arguments and dependencies. Sassdocs annotations are preferred.

For a Sass projects main file a table of contents should be added at the top of the file for reference of all partials within the project. This should be updated whenever a partial is added e.g.

//
//  CONTENTS
// 
//  UTILITIES
//  Functions............Useful functions.
//  Mixins
//      Breakpoint.......Media query mixin.
//      Rem..............Conditionally output px or rem.
//      Before IE 10.....Conditionally output content for pre IE10.
//      Before IE 9......Conditionally output content for pre IE9.
//      Bourbon..........Bourbon Mixin library - See http://bourbon.io/docs/

Sass

###Imports Never use inline imports. For imports inside .scss files do not use underscores or filename extensions.

// Don't do this
@import '_form-list.scss';

// Do this
@import 'form-list';

###Values Reusable values should be stored in variables without a unit attached.

// Don't do this
$vertical-spacing: 20px;

// Do this
$vertical-spacing: 20;

###Calculations When performing Math in Sass always enclose them in parentheses.

// Don't do this
width: $container-width / 2;

// Do this
width: ($container-width / 2);

###Maps Maps should be used whenever values will be reused repeatedly. Common examples include breakpoints, colour groups and z-index values. Maps should have the following syntax:

Hyphenated name
Single quoted keys
Space after the colon
Opening brace on same line as map name
Key/Value pairs on same line
Comma at end of each key/value pair except the last
Closing brace on it’s own line

$breakpoints: (
    'small': 30rem,
    'medium': 48.125rem,
    'large': 75rem
);

These can then be fetched through the map-get function:

map-get($breakpoints, 'small')

###Variable Naming Names should not be based on buzzwords or physical objects. They should be developer friendly and understandable. Colour names should be representative of themselves. If colour names are difficult to remember, additional variables can be created that reference the original colours.

// Do this
$brand-colours: (
    'teal': #088,
    'royal-blue': #4169E1 
);

$divider-colour = map-get($brand-colours, 'teal');

###Nesting Selector nesting should be kept as simplistic as possible. Attempting to be clever (although impressive) will make sass harder to read and take longer to analyze. Nesting should also be kept to a maximum of 3 levels deep. If a selector needs to be more than 3 levels deep then refactoring is required. There is always a way to reduce the specificity and still target the desired elements.

// Don't do this
.navigation {
    ul {
        >li {
            &:first-child {
                margin-left: 0;
            }
        }
    }
}

// Be terse
.navigation-parent-item {
    &:first-child {
        margin-left: 0;
    }
}

###Animations, Mixins & Functions Each mixin, function and animation should be separated out into it’s own partial and documented using SassDoc annotations.

###Extend @extend should be reserved for use on sass placeholders only.

###Sass Declarations In sass rulesets declarations should be in the following order:

Extends
Includes without content
Normal declarations
Includes with content
Nested rule sets

###Media Queries Use an existing mixin for declaring a media query and place it nested inside the ruleset. This allows the developer to easily find related media queries. Optimisation for duplicate media queries can be dealt with as a performance task.

.selector {
    @include linear-gradient(to bottom, #fff, #f2f2f2, $fallback: #fff);

    @include breakpoint(map-get($breakpoints, 'medium')) {
        background-color: inherit;
        background-image: none;
    }
}

Legacy Browsers

Sass allows handling of legacy browser specific css without the use of hacks. Hacks should never be used under any circumstance. For old IE browser versions, a separate css file should be created with fallbacks. This ensures that modern browsers are not populated with unnecessary declarations. In Sass projects use the IE mixin:

.selector {
    background-color: rgba(255, 255, 255, .6);
    
    @include before-ie-9 {
        background-image: url('images/white-transparency-60.png');
    }
}

Meanbee

Coding Standards for Magento