Principles of writing consistent, idiomatic CSS

The following document outlines a reasonable style guide for CSS development. These guidelines strongly encourage the use of existing, common, sensible patterns. They should be adapted as needed to create your own style guide.

This is a living document and new ideas are always welcome. Please contribute.

General principles
Whitespace
Comments
Format
Practical example

Acknowledgements

License

1. General principles

"Part of being a good steward to a successful project is realizing that writing code for yourself is a Bad Idea™. If thousands of people are using your code, then write your code for maximum clarity, not your personal preference of how to get clever within the spec." - Idan Gazit

You are not a human code compiler/compressor, so don't try to be one.
All code in any code-base should look like a single person typed it, no matter how many people contributed.
Strictly enforce the agreed-upon style.
If in doubt when deciding upon a style, use existing, common patterns.

2. Whitespace

Only one style should exist across the entire source of your code-base. Always be consistent in your use of whitespace. Use whitespace to improve readability.

Never mix spaces and tabs for indentation.
Choose between soft indents (spaces) or real tabs. Stick to your choice without fail. (Preference: spaces)
If using spaces, choose the number of characters used per indentation level. (Preference: 4 spaces)

Tip: configure your editor to "show invisibles". This will allow you to eliminate end-of-line whitespace, eliminate unintended blank-line whitespace, and avoid polluting commits.

Tip: use an EditorConfig file (or equivalent) to help maintain the basic whitespace conventions that have been agreed for your code-base.

3. Comments

Well commented code is extremely important. Take time to describe components, how they work, their limitations, and the way they are constructed. Don't leave others in the team guessing as to the purpose of uncommon or non-obvious code.

Comment style should be simple and consistent within a single code base.

Place comments on a new line above their subject.
Keep line-length to a sensible maximum, e.g., 80 columns.
Make liberal use of comments to break CSS code into discrete sections.
Use "sentence case" comments and consistent text indentation.

Tip: configure your editor to provide you with shortcuts to output agreed-upon comment patterns.

Example:

/* ==========================================================================
   Section comment block
   ========================================================================== */

/* Sub-section comment block
   ========================================================================== */

/**
 * Short description using Doxygen-style comment format
 *
 * The first sentence of the long description starts here and continues on this
 * line for a while finally concluding here at the end of this paragraph.
 *
 * The long description is ideal for more detailed explanations and
 * documentation. It can include example HTML, URLs, or any other information
 * that is deemed necessary or useful.
 *
 * @tag This is a tag named 'tag'
 *
 * @todo This is a todo statement that describes an atomic task to be completed
 *   at a later date. It wraps after 80 characters and following lines are
 *   indented by 2 spaces.
 */

/* Basic comment */

4. Format

The chosen code format must ensure that code is: easy to read; easy to clearly comment; minimizes the chance of accidentally introducing errors; and results in useful diffs and blames.

Use one discrete selector per line in multi-selector rulesets.
Include a single space before the opening brace of a ruleset.
Include one declaration per line in a declaration block.
Use one level of indentation for each declaration.
Include a single space after the colon of a declaration.
Use lowercase and shorthand hex values, e.g., #aaa.
Use single or double quotes consistently. Preference is for double quotes, e.g., content: "".
Quote attribute values in selectors, e.g., input[type="checkbox"].
Where allowed, avoid specifying units for zero-values, e.g., margin: 0.
Include a space after each comma in comma-separated property or function values.
Include a semi-colon at the end of the last declaration in a declaration block.
Place the closing brace of a ruleset in the same column as the first character of the ruleset.
Separate each ruleset by a blank line.

.selector-1,
.selector-2,
.selector-3[type="text"] {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
    display: block;
    font-family: helvetica, arial, sans-serif;
    color: #333;
    background: #fff;
    background: linear-gradient(#fff, rgba(0, 0, 0, 0.8));
}

.selector-a,
.selector-b {
    padding: 10px;
}

Declaration order

If declarations are to be consistently ordered, it should be in accordance with a single, simple principle. My preference is for structurally important properties (e.g. positioning and box-model) to be declared prior to all others.

.selector {
    /* Positioning */
    position: absolute;
    z-index: 10;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;

    /* Display & Box Model */
    display: inline-block;
    overflow: hidden;
    box-sizing: border-box;
    width: 100px;
    height: 100px;
    padding: 10px;
    border: 10px solid #333;
    margin: 10px;

    /* Other */
    background: #000;
    color: #fff;
    font-family: sans-serif;
    font-size: 16px;
    text-align: right;
}

Strict alphabetical ordering is also relatively popular, but the drawback is that it separates related properties. For example, position offsets are no longer grouped together and box-model properties can end up spread throughout a declaration block.

Exceptions and slight deviations

Large blocks of single declarations can use a slightly different, single-line format. In this case, a space should be included after the opening brace and before the closing brace.

.selector-1 { width: 10%; }
.selector-2 { width: 20%; }
.selector-3 { width: 30%; }

Long, comma-separated property values - such as collections of gradients or shadows - can be arranged across multiple lines in an effort to improve readability and produce more useful diffs. There are various formats that could be used; one example is shown below.

.selector {
    background-image:
        linear-gradient(#fff, #ccc),
        linear-gradient(#f3c, #4ec);
    box-shadow:
        1px 1px 1px #000,
        2px 2px 1px 1px #ccc inset;
}

Preprocessors: additional format considerations

Different CSS preprocessors have different features, functionality, and syntax. Your conventions should be extended to accommodate the particularities of any preprocessor in use. The following guidelines are in reference to Sass.

Limit nesting to 1 level deep. Reassess any nesting more than 2 levels deep. This prevents overly-specific CSS selectors.
Avoid large numbers of nested rules. Break them up when readability starts to be affected. Preference to avoid nesting that spreads over more than 20 lines.
Always place @extend statements on the first lines of a declaration block.
Where possible, group @include statements at the top of a declaration block, after any @extend statements.
Consider prefixing custom functions with x- or another namespace. This helps to avoid any potential to confuse your function with a native CSS function, or to clash with functions from libraries.

.selector-1 {
    @extend .other-rule;
    @include clearfix();
    @include box-sizing(border-box);
    width: x-grid-unit(1);
    // other declarations
}

5. Practical example

An example of various conventions.

/* ==========================================================================
   Grid layout
   ========================================================================== */

/**
 * Column layout with horizontal scroll.
 *
 * This creates a single row of full-height, non-wrapping columns that can
 * be browsed horizontally within their parent.
 *
 * Example HTML:
 *
 * <div class="grid">
 *     <div class="cell cell-3"></div>
 *     <div class="cell cell-3"></div>
 *     <div class="cell cell-3"></div>
 * </div>
 */

/**
 * Grid container
 * Must only contain `.cell` children.
 */

.grid {
    height: 100%;
    /* Remove inter-cell whitespace */
    font-size: 0;
    /* Prevent inline-block cells wrapping */
    white-space: nowrap;
}

/**
 * Grid cells
 * No explicit width by default. Extend with `.cell-n` classes.
 */

.cell {
    position: relative;
    display: inline-block;
    overflow: hidden;
    box-sizing: border-box;
    height: 100%;
    /* Set the inter-cell spacing */
    padding: 0 10px;
    border: 2px solid #333;
    vertical-align: top;
    /* Reset white-space */
    white-space: normal;
    /* Reset font-size */
    font-size: 16px;
}

/* Cell states */

.cell.is-animating {
    background-color: #fffdec;
}

/* Cell dimensions
   ========================================================================== */

.cell-1 { width: 10%; }
.cell-2 { width: 20%; }
.cell-3 { width: 30%; }
.cell-4 { width: 40%; }
.cell-5 { width: 50%; }

/* Cell modifiers
   ========================================================================== */

.cell--detail,
.cell--important {
    border-width: 4px;
}

Translations

Acknowledgements

Thanks to everyone who has provided translations and to all those who contributed to idiomatic.js. It was a source of inspiration, quotations, and guidelines.

License

Principles of writing consistent, idiomatic CSS by Nicolas Gallagher is licensed under the Creative Commons Attribution 3.0 Unported License. This applies to all documents and translations in this repository.

Based on a work at github.com/necolas/idiomatic-css.

JayMcDowell/idiomatic-css

Principles of writing consistent, idiomatic CSS

Table of contents

1. General principles

2. Whitespace

3. Comments

4. Format

Declaration order

Exceptions and slight deviations

Preprocessors: additional format considerations

5. Practical example

Translations

Acknowledgements

License