CSS Style Guide
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CSS Style Guide

This styleguide addresses both the CSS syntax specifics and an actual model of how to structure the CSS of a web application, in an organized scalable way.


  • Use UTF-8.

  • Use 2 space indent, no tabs.

  • Avoid trailing whitespace.

  • Use a line for each selector when using multiple comma-separated selectors.

  • Include a space before the opening brace.

  • Leave the closing brace on its own line, leaving an empty line afterwards.

  • Use one space after : for each declaration, and end every declaration with a ;.

  • Put every property declaration in a single line, with the exception of single declarations.

    /* Bad */
    .navTab {
      margin: 20px; padding: 20px;
    /* Good */
    .navTab {
      margin: 20px;
      padding: 20px;
    /* Good */
    .icon--plus { background-position: -4px -28px; }
    .icon--minus { background-position: -4px -58px; }
  • Align vendor prefixed properties with the non-prefixed property.

    -webkit-box-shadow: 0 1px 2px rgba(0, 0, 0, 0.15);
       -moz-box-shadow: 0 1px 2px rgba(0, 0, 0, 0.15);
         -o-box-shadow: 0 1px 2px rgba(0, 0, 0, 0.15);
            box-shadow: 0 1px 2px rgba(0, 0, 0, 0.15);
  • Use rgb() and rgba() only to define colors, and don't leave spaces after the opening parentheses or before the closing parentheses.

  • Use a space after every comma.

  • Use a preceding zero for decimal numbers.

  • Use double quotes for values in attribute value selectors (e.g. input[type="text"]).

  • Don't specify units for zero values.

  • Declare properties in alphabetical order, declaring mixins first if using LESS/SASS.

  • Don't use @import, add an extra <link> tag instead.

  • Prefer using the shorthand form of properties to declaring their individual values separately.

    /* Bad */
    .someElement {
      margin-bottom: 6px;
      margin-left: 5px;
      margin-right: 7px;
      margin-top: 8px;
    /* Good */
    .someElement {
      margin: 8px 7px 6px 5px;
  • Try to shorten the margin and padding when possible to their shorthand notation, one-value or two-value form. Don't use its shorter three-value form.

  • Place media queries close to the elements they are referring to, don't put them all at the end of the document.

  • Try to avoid using IDs as part of the selectors.

  • Try to avoid nesting as much as possible, don't use more than 3 levels of nesting.

  • Use em instead of px or pt for font-size definitions. And no unit for line-height definitions (e.g. 1.4). Except for when the line-height has to match the fixed box height, in which case the line-height is the elements height minus 1.

  • Prefer selecting elements through class names, instead of element name or through attributes.

  • Use double quotes for properties that receive strings, like font-family values and url("").

  • Never use any rules with !important.


  • Don't use class names that specify presentational behavior, instead use class names that describe a semantic structure. Grid-like class names (col-6, row) shouldn't never be used.

  • Compile your assets with a preprocessor like LESS or SASS into a single file.

  • Use a reset file, to avoid browser implementation inconsistencies.

  • Use comments to separate a cohesive declaration of rule sets including multiple *. Each of these comment lines should have 80 characters in size.

  • Avoid too long or too short class names.

  • Keep the code simple.

  • Be consistent.

  • Use common sense.

LESS (or SASS) specifics

  • Use mixins for every property declaration that requires vendor prefixes, mixin names are prefixed with m-.

  • Use a mixin for declaring the font-size and line-height in px, to transform it into em, receiving the context in px as an optional parameter.

  • Avoid nesting selectors as much as possible, unless nesting the selectors is your only choice.

CSS architecture using BEM

The BEM approach to structure the CSS has been proven to be the most successful in organizing and working with presentational assets in web applications.

The BEM (Block-Element-Modifier) approach consists in separating every piece of the UI into components, so that they can be reused throughout the entire application. These components would naturally derive from a styleguide where every component is defined separately. It is only through the composition of these components that the entire pages are built.

It is recommended that each of these components are specified in a different file and then compiled into a single stylesheet.

The BEM convention used here consists of declaring the component, its descendants and modifiers as follows:

Component (Block)

The component name consists of a camelCase declaration which is descriptive according to the structure of the component.

.navMenu {}

Descendants (Element)

The component descendants consist of the child elements of the component. They are named using camelCase as well, and prepended by the component name separated by a hyphen.

.navMenu-item {}


Modifiers apply when the presentation of the main component or its descendants changes slightly from its original one. These rules are specified after the component or descendant specification. The name consists of the component or descendant name, two hyphens and the camelCase modifier name.

.navMenu--secondary {}
.navMenu-item--featured {}

State declarations

State declarations on components or descendants are used to indicate some state of the element itself. Every state indicator is prepended by is-. Common state declarations used are: .is-active, .is-disabled, .is-expanded.

.navMenu {
  display: none;

.navMenu.is-expanded {
  display: block;

Javascript classes

Use js- prefixed class names for elements that are going to be manipulated through JavaScript. Examples: js-tooltip, js-dropdown.

Folder structure

A typical folder structure to hold this architecture would be:

  • On the LESS stylesheets root should be a main file for each CSS file to be generated as output. Usually, you could have application.less, admin.less and mobile.less.

  • Then there's the components/ folder, where you have every component with one file per component, using the component name as the file name. Each of these files only contains the component definition, including its descendants and modifiers.

  • The there's the shared/ folder, that contains:

    • The base.less file, which specifies a few base styles for the entire HTML document, treat the base file as if there was a base component, where you define the font-size, background-color, etc for the document/body.
    • The mixins.less file, which specifies all the mixins to be used by the components.
    • The reset.less file, which sets the base styles for all HTML elements to maintain a consistency between the browsers.
    • The variables.less file, which holds all the variables to be used by the components and mixins.
├-- application.less
├-- mobile.less
├-- admin.less
├-- components/
│   ├-- action.less
│   ├-- detailCard.less
│   ├-- formField.less
│   ├-- icon.less
│   ├-- list.less
│   └-- selectMenu.less
└-- shared/
    ├-- base.less
    ├-- mixins.less
    ├-- reset.less
    └-- variables.less

public/ # Output folder
└-- stylesheets/
    ├-- application.css
    ├-- mobile.css
    └-- admin.css


Variables should be used for everything that could be reused. Use variables for the following things:

  • z-index, this should go from the lowest z-index value (100) to the highest (1000), use them in order of availability and try to avoid reaching the highest value.

/* z-index definition */ @zIndex-1: 100; @zIndex-2: 200; @zIndex-3: 300; @zIndex-4: 400; @zIndex-5: 500; @zIndex-6: 600; @zIndex-7: 700; @zIndex-8: 800; @zIndex-9: 900; @zIndex-10: 1000;

/* z-index application */ @sidebar--zIndex: @zIndex-1;

@mainHeader--zIndex: @zIndex-2; @caption--zIndex: @zIndex-2; @tooltip--zIndex: @zIndex-2;

* `font-weight`, which should be defined as follows:
/* font-weight definition */
@fontWeight-1: 200;
@fontWeight-2: 400;
@fontWeight-3: 700;
@fontWeight-4: 900;

/* font-weight application */
@mainHeader-title--fontWeight: @fontWeight-4;
  • letter-spacing, which should be defined as follows:

/* letter-spacing definition */ @letterSpacing-tighter: -0.1em @letterSpacing-tight: -0.04em; @letterSpacing-normal: -0.02em; @letterSpacing-loose: 0; @letterSpacing-looser: 0.02em;

/* letter-spacing application */ @tooltip--letterSpacing: @letterSpacing-tight;

* Base `font-size`, `line-height`, `font-family`. These are then used on the
/* fonts */
@base--fontSize: 13px;
@base--lineHeight: 1.4em;
@base--fontFamily: "Helvetica Neue", "Helvetica", Arial;
  • Colors. Not all colors are supposed to be specified in variables to then be used on the components. Usually the colors that are important and define the look and feel of the pages are the ones that need to go in the shared/variables.less. The format for specifying these colors would be <component>[-descendant][-modifier]--<property>. Some examles would be:

    /* colors */
    @base--color: rgb(56, 56, 56);
    @base--backgroundColor: rgb(255, 255, 255);
    @mainHeader--backgroundColor: rgb(42, 52, 64);
    @navMenu-item--color: rgba(0, 0, 0, 0.9);
    @navMenu-item-active--color: rgba(0, 136, 204, 0.9);

    Things that would not need go in the shared/variables.less are for example box shadow colors and other neutral colors that wouldn't change regardless of the site style.

    Use as few colors as possible, and try to share them in multiple components. If necessary, use LESS color transformation functions to reduce the opacity of an existing color and to lighten it up.


Using a system of grids with classes is the opposite from what you want to achieve using CSS classes. One of the main purposes of using CSS is to remove the presentation aspect from the HTML. Classes like col-3 and span-4 are purely presentational, and they violate the purpose of using stylesheets and separating presentation from the semantic structure.

Every layout of every page can also be abstracted into the sum of multiple components. Usually these components will consist of some header, sidebar, content, footer and maybe even secondary headers and a bunch of box groups.

An example of a layout component, would be .mainHeader, which could be defined as follows:

/* components/mainHeader.less */
/* mainHeader *****************************************************************/
.mainHeader {
  background: @mainHeader--background-color;
  height: 80px;
  line-height: 79px;

.mainHeader-title {
  display: none;

.mainHeader-logo {
  float: left;
  margin: 20px 0;
  max-height: 40px;

.mainHeader-nav {
  float: right;

/* components/contentWrapper.less */
/* contentWrapper *************************************************************/
.contentWrapper {
  margin: 0 auto;
  max-width: 520px;
<header class="mainHeader">
  <div class="contentWrapper">
    <a href="/">
      <h1 class="mainHeader-title">App Name</h1>
      <img class="mainHeader-logo" src="/images/app-logo.png"
           alt="App Logo" />
    <nav class="mainHeader-nav">
      <ul class="navMenu">
        <li class="navMenu-item is-active"><a href="/">Home</a></li>
        <li class="navMenu-item"><a href="/overview">Overview</a></li>


The mobile approach depends solely on the web application size, and how it is expected for it to scale.

If the webapp is a small one which will remain this way the responsive design is the way to go. You deal with this by using media queries and fluid layouts.

If the webapp is a large scale application, then using an adaptive approach by detecting the agent from the server-side and rendering the necessary data depending on the device is the best approach. This will reduce the difficulty needed to be able to take into account multiple devices like the responsive approach does. This approach consists of building one webapp for the large screens and a different one for mobile. These two applications would share most of the components and variables usage, but there will be some differences. This approach, will most likely reduce the page load time and will accelerate the development process.