CSS Coding Guide

Coding style can be really personal and everyone has their own opinions and preferences. But when we work as a team on a shared codebase it’s invaluable to agree to some basic rules. The CSS in Protocol should be written consistently no matter who wrote it.

Terminology

Just so we all know what we’re talking about, a CSS rule comprises one or more selectors followed by a declaration block consisting of one or more declarations. A declaration comprises a property and a value (some properties accept multiple values).

A rule in CSS looks like:

selector {
    property: value;
}

Simple selectors

Use the shortest, least specific selector required to do the job.

Favor classes over IDs. IDs in CSS aren’t expressly forbidden, just strongly discouraged. Using ID selectors can lead to specificity wars requiring ever more powerful selectors to override previous styling. A better option is an attribute selector like [id='widget'] which selects the element by its ID but has the same specificity as a class. Everybody wins.

Avoid qualifying classes with type selectors. It slows down performance and makes classes less portable. E.g. .widget is better than div.widget.

Minimal nesting

We use Sass as a pre-processor. Nested rules are converted into descendent selectors in the generated style sheet. The deeper the nesting, the more complex and specific the final selector will be. Don’t nest rules unless necessary for context and specificity, and don’t nest rules just to group them together (use comments to label sections of the style sheet for grouping).

All the style declarations for the parent element should come before any nested rules.

Include a blank line before each nested rule to separate it from the rule or declaration above it.

// NO - This is horrible
.widget-container {
    float: right;
    .widgets {
        .widget-foo {
            background: #ccc;
            h3 {
                color: red;
            }
            padding: 10px;
        }
    }
    width: 30%;
}

// YES - Clean and simple
.widget-container {
    float: right;
    width: 30%;
}

.widget-foo {
    background: #ccc;
    padding: 10px;

    h3 {
        color: red;
    }
}

Format

  • One selector per line.
  • One declaration per line.
  • Order declarations alphabetically (from A to Z).
  • Use a soft indent of four spaces - don’t use tabs.
  • Use one space between the selector and the first bracket.
  • Use one space between property and value after :.
  • Always add a semicolon after property value.
  • Use single quotes.
  • Do not specify units for a zero value.
  • Omit the leading zero in decimal values, e.g. .75em not 0.75em.
  • Include a space after each comma in a comma-separated property list.
  • User lowercase and shorthand hex values, e.g. #aaa.
  • Always use hex values unless you are declaring rgba.
  • Separate each rule by a blank line.
  • Use // for comments (instead of /* */).
.selector1,
.selector2 {
    // This is a comment
    background: #333 url('img/icon.png') center no-repeat;
    color: #bada55;
    margin: 0 auto .75em;
}

.selector-a,
.selector-b {
    background: rgba(255, 255, 255, .25);
    padding: 20px;
}

When possible, limit line lengths to 80 characters. It improves readability, minimizes horizontal scrolling, makes it possible to view files side by side, and produces more useful diffs with meaningful line numbers. There will be exceptions such as long URLs or gradient syntax but most declarations should fit well within 80 characters even with indentation.

Long, comma-separated property values – such as multiple background images, gradients, transforms, transitions, webfonts, or text and box shadows – can be arranged across multiple lines (indented one level from their property).

.selector {
    background-image:
        linear-gradient(#fff, #ccc),
        linear-gradient(#f3c, #4ec);
    box-shadow:
        1px 1px 1px #000,
        2px 2px 1px 1px #ccc inset;
    transition:
        border-color 500ms ease-in,
        opacity 100ms ease-in;
}

Comments

Use // for comments in Sass instead of /* */, including multi-line comment blocks. Our configuration is set to strip // comments when the CSS is compiled and minified, but will allow /* */ comments to pass through. There’s usually no need to include developer comments in minified CSS sent to browsers, so it just adds extra bytes.

An exception is special comments to override linting rules, which require the /* */ formatting.

// This thing should never
// ever have a border.
.this-thing {
    border: 0 none !important; /* stylelint-disable-line declaration-no-important */
}

In a long style sheet it’s not a bad idea to add dividing lines (with two blank lines before it) to help set sections apart visually.

//*----------------------------------------------------------------------------*/
// Useless Sass Function

// This is an imaginary function that doesn't actually do
// anything, but it does include an example of how to format
// multi-line comment blocks.
// Usage:
//    non-function('non-value');

@function non-function($non-value) {
    @if $non-value {
        @return $non-value;
    }

    @return $non-value;
}

Units

  • Use pixels for fixed-width elements.
  • Use percentages for fluid-width elements.
  • Use rems for font-size because it respects user preferences.
  • Use unitless line-height in conjunction with font-size; it acts as a multiplier of font size. E.g. line-height: 1.5.
  • Use milliseconds for timing, e.g. 500ms not .5s.