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CSS Structure and Rules

Spacing

  • Soft tabs, 4 spaces
  • Put spaces before { in rule declarations.
  • Put spaces after : in property declarations.
  • End property declarations with ;.
  • Put line breaks between rulesets.
  • Place closing braces of declaration blocks on a new line.
  • A blank line should be placed between each selector block.
.foo {
	property: value;
	property: value;
}

.bar {
	property: value;
}

Large blocks of single declarations can use a one-line format.

.icon_arrow::before { content: "\e602"; }
.icon_calendar::before { content: "\e603"; }
.icon_clock::before { content: "\e604"; }

Long, comma-separated property values, such as collections of gradients or shadows, can be arranged across multiple lines. It improves readability and produces more useful diffs.

.foo {
    box-shadow:
        1px 1px 1px #000,
        1px 1px 1px 1px #333 inset;
}

Formatting

  • Only use lowercase (except for comments)
  • Use shorthand hex values, when possible. Use #fff instead of #ffffff
  • Use double quotes "..." around values

Units

  • line-height should be unit-less, thus making it a multiplier based on the font-size
  • Avoid specifying units for zero values. Use margin: 0; instead of margin: 0px;

Property Ordering

Properties are grouped by categories. Each category is separated with a blank line.

  • Content
  • content
  • Typography, color and background
  • background color filter font letter line-height text word
  • Display
  • border box-sizing display height margin max-height min-height outline overflow padding vertical-align visibility width
  • Flex (display: flex must be first of block)
  • align flex justify-content
  • Positioning
  • bottom float left position right top z-index
  • Visual and animation
  • animation box-shadow opacity transform
  • Others
  • appearance list-style table-style cursor ...
.element {
	color: #000;
	font-family: Arial, sans-serif;
    font-size: 1.5rem;

	display: block;
	padding: 12px 20px 9px;
	margin-left: 0;
	border-bottom: 1px solid #333;

	position: absolute;
	top: 10em;
	left: 10em;

	@mixin transition(#333, .2s);
}

Selectors

  • Reduce location depedency
  • Increase portability. Use .widget__link instead of .widget > a
  • Avoid over-qualified selectors. Use .widget__link instead of a.widget__link
  • JS hooks prefixed with js- : .js-menu
  • State prefixed with is- : .is-open
  • Try to avoid key selectors (the right-most part of a selector) that are too large, like .foo div. When it's impossible to avoid, try to use a child selector (e.g. .foo > div). This will limit the browser to look for only one level higher in the DOM

Naming Convention

Class names should be human readable, communicating useful information. Use structural and purposeful names over presentational ones (e.g. .important instead of .red). Be short but as specific as possible. Go for .site-logo instead of .logo.

BEM

We use a version of the BEM technique to add intent and reletionship to classnames :

  • .block
  • .block__element
  • .block--modifier
<div class="post">
	<h2 class="post__title">My post!</h2>
	<img class="post__image post__image--large" />
	<img class="post__image" />
	<img class="post__image" />
</div>

The name of a block, element or modifier consisting of multiple words will use the - delimiter. A block named primary nav would become .primary-nav.

Specificity

Keep CSS specificity as low as possible.

Single Responsibility Principle

Each class must serve a single responsibility, decomposing big blocks into small pieces that do only one thing, but do it well. Those pieces can then be reused into other elements.

Every time you see a repeatable pattern between multiple classes, it should be splitted into a new class that will handle this pattern only.

Bad example:

<a class="card">Simple card #1</a>
<a class="card">Simple card #2</a>
<a class="ad-spot">This is an ad spot!</a>

<style>
	.card {
		background-color: #fff;

		display: block;
		width: 300px;
		padding: 20px;

		border-radius: 5px;
	}

	.ad-spot {
		background-color: #ccc;

		display: block;
		width: 300px;
		padding: 20px;
	}
</style>

This is bad, because .card and .adSpot are sharing common styling. If we need to change the width of a card, we will also need to change it inside the ad spot CSS.

Good example:

<a class="box card">Simple card #1</a>
<a class="box card">Simple card #2</a>
<a class="box ad-spot">This is an ad spot!</a>

<style>
	.box {
		display: block;
		width: 300px;
		padding: 20px;
	}

	.card {
		background-color: #fff;

		border-radius: 5px;
	}

	.ad-spot {
		background-color: #ccc;
	}
</style>

We now see that the .box class is reusable for any items. As this is only doing one thing (setting the display), we can combine it with other classes to build something more complex.

Variables

Types

  • Color, starting with c-
  • Font, starting with f-
  • Size, starting with s-

Naming Convention

  • Lowercase
  • Use the - delimiter
  • You can use the modifier -- like in BEM
  • The first letter is about the type of the variable

Examples

  • c-primary : Primary color
  • c-secondary--light : Secondary color, lighter
  • f-primary--light : Primary font with bold weight
  • s-lineheight : Default line-height value

Comments

Each CSS file should begin with it's name and a short description of its main purpose.

/**
 * STYLE.CSS
 * Default application style
 */

Table of contents

A table of content, just after the initial file declaration, should be describing the main sections of the file. With larger files, adding sub-sections can also be useful.

/**
 * STYLE.CSS
 * Default application style
 *
 * - REQUIRE - DEF              Require and images def
 * - INIT                       Document initialization
 * - LAYOUT                     Container, Wrapper, Stripe
 * - SITE SECTIONS              Site sections: sidebar, nav, header, main
 * - ELEMENTS                   Default style for elements like <p> and <a>
 * - FORMS                      Form styling
 * - MESSAGES                   Messages
 * - STATES                     States
 * - CLASSES                    Common classes
 */

or

/**
 * LINECHART.CSS
 * Linechart for DC elements
 *
 * - REQUIRE - DEF              Require and images def
 * - STYLE
 *   + Legend
 *   + Axis 
 *   + Grid
 */

Sections

Every sections (defined in the table of content) should start with a title. This title is precided by 2 new lines and have one new line after.

/* -- Name of the section ---------------------------------------------- */

Sub-sections

Useful sub-sections should also be delimited by a simpler title.

/* -> My sub-section -- */

Media-queries

Place media queries as close to their relevant rule sets whenever possible. Don't bundle them all in a separate stylesheet or at the end of the document. Doing so only makes it easier for folks to miss them in the future. Here's a typical setup.

.element { ... }
.element--selected { ... }
.element__avatar { ... }

@media (min-width: 480px) {
	.element { ...}
	.element--selected { ... }
	.element__avatar { ... }
}

Others

  • W3C valid
  • Add new line at end of file
  • Classes, variables and comments should use english spelling