CSS Style Guide

This skill applies Google's CSS style guide conventions to ensure clean, maintainable, and efficient CSS code.

Core Principles

Valid CSS

Use valid CSS code tested with W3C CSS Validator:

• Catches errors early
• Ensures browser compatibility
• Improves maintainability
• Exception: vendor-specific prefixes and required proprietary syntax

Meaningful Class Names

Use functional or generic class names, not presentational:

/* Not recommended: presentational */
.button-green {}
.clear {}
.yee-1901 {} /* meaningless */

/* Recommended: functional */
.gallery {}
.login {}
.video {}
.aux {} /* generic helper */
.alt {} /* generic alternative */

Avoid ID Selectors

IDs have high specificity and reduce reusability:

/* Not recommended */
#example {}
#navigation {}

/* Recommended */
.example {}
.navigation {}

CSS Style Rules

Class Naming Conventions

Use short but meaningful names:

/* Not recommended */
.navigation {}
.atr {}

/* Recommended */
.nav {}
.author {}

Use hyphens as delimiters:

/* Not recommended */
.demoimage {}
.error_status {}

/* Recommended */
.video-id {}
.ads-sample {}

Use prefixes for large projects (optional):

/* For namespacing in large projects */
.adw-help {} /* AdWords */
.maia-note {} /* Maia */

Avoid Type Selectors

Don't qualify class names with type selectors:

/* Not recommended */
ul.example {}
div.error {}

/* Recommended */
.example {}
.error {}

Reason: Performance and flexibility

Shorthand Properties

Use shorthand when possible:

/* Not recommended */
border-top-style: none;
font-family: palatino, georgia, serif;
font-size: 100%;
line-height: 1.6;
padding-bottom: 2em;
padding-left: 1em;
padding-right: 1em;
padding-top: 0;

/* Recommended */
border-top: 0;
font: 100%/1.6 palatino, georgia, serif;
padding: 0 1em 2em;

Units

Omit units after 0 values:

/* Not recommended */
margin: 0px;
padding: 0em;

/* Recommended */
margin: 0;
padding: 0;

/* Exception: required units */
flex: 0px; /* flex-basis requires unit */
flex: 1 1 0px; /* needed in IE11 */

Include leading 0s:

/* Not recommended */
font-size: .8em;

/* Recommended */
font-size: 0.8em;

Color Values

Use 3-character hex when possible:

/* Not recommended */
color: #eebbcc;

/* Recommended */
color: #ebc;

Use lowercase:

/* Not recommended */
color: #E5E5E5;

/* Recommended */
color: #e5e5e5;

Important Declarations

Avoid !important - use specificity instead:

/* Not recommended */
.example {
  font-weight: bold !important;
}

/* Recommended */
.example {
  font-weight: bold;
}

/* If override needed, increase specificity */
.container .example {
  font-weight: bold;
}

Browser Hacks

Avoid CSS hacks and user agent detection - use progressive enhancement:

/* Not recommended */
.example {
  width: 100px\9; /* IE hack */
}

/* Recommended: Use feature queries */
@supports (display: grid) {
  .example {
    display: grid;
  }
}

Formatting Rules

Indentation

Indent by 2 spaces (no tabs):

.example {
  color: blue;
}

@media screen, projection {

  html {
    background: #fff;
    color: #444;
  }

}

Declaration Order

Alphabetize declarations for consistency (optional):

/* Recommended */
background: fuchsia;
border: 1px solid;
-moz-border-radius: 4px;
-webkit-border-radius: 4px;
border-radius: 4px;
color: black;
text-align: center;
text-indent: 2em;

Note: Ignore vendor prefixes for sorting, but keep them grouped

Semicolons

Always end declarations with semicolons:

/* Not recommended */
.test {
  display: block;
  height: 100px
}

/* Recommended */
.test {
  display: block;
  height: 100px;
}

Property-Value Spacing

Single space after colon, no space before:

/* Not recommended */
h3 {
  font-weight:bold;
}

/* Recommended */
h3 {
  font-weight: bold;
}

Declaration Block Spacing

Single space before opening brace, same line:

/* Not recommended: missing space */
.video{
  margin-top: 1em;
}

/* Not recommended: unnecessary line break */
.video
{
  margin-top: 1em;
}

/* Recommended */
.video {
  margin-top: 1em;
}

Selector and Declaration Separation

New line for each selector and declaration:

/* Not recommended */
a:focus, a:active {
  position: relative; top: 1px;
}

/* Recommended */
h1,
h2,
h3 {
  font-weight: normal;
  line-height: 1.2;
}

Rule Separation

Blank line between rules:

html {
  background: #fff;
}

body {
  margin: auto;
  width: 50%;
}

Quotation Marks

Use single quotes for attribute selectors and property values:

/* Not recommended */
@import url("https://www.google.com/css/maia.css");

html {
  font-family: "open sans", arial, sans-serif;
}

/* Recommended */
@import url(https://www.google.com/css/maia.css);

html {
  font-family: 'open sans', arial, sans-serif;
}

/* Exception: @charset requires double quotes */
@charset "utf-8";

Do not quote URLs:

/* Recommended */
background: url(images/bg.png);

Organizing CSS

Section Comments

Group related rules with comments (optional):

/* Header */

.adw-header {}

.adw-header-logo {}

/* Footer */

.adw-footer {}

/* Gallery */

.adw-gallery {}

.adw-gallery-item {}

File Organization

Organize CSS files logically:

/* Base styles */
html,
body {
  margin: 0;
  padding: 0;
}

/* Typography */
h1, h2, h3 {
  font-family: 'Arial', sans-serif;
}

/* Layout */
.container {
  max-width: 1200px;
  margin: 0 auto;
}

/* Components */
.button {
  padding: 10px 20px;
}

/* Utilities */
.hidden {
  display: none;
}

Best Practices

Specificity Management

Keep specificity low for easier overrides:

/* Not recommended: too specific */
html body div.container ul.nav li.item a.link {}

/* Recommended */
.nav-link {}

Avoid Over-nesting

/* Not recommended */
.header .nav .menu .item .link {
  color: blue;
}

/* Recommended */
.nav-link {
  color: blue;
}

Mobile-First Media Queries

/* Base styles for mobile */
.container {
  width: 100%;
}

/* Progressively enhance for larger screens */
@media (min-width: 768px) {
  .container {
    width: 750px;
  }
}

@media (min-width: 1024px) {
  .container {
    width: 960px;
  }
}

Reusable Classes

Create utility classes for common patterns:

/* Layout utilities */
.flex {
  display: flex;
}

.flex-center {
  display: flex;
  justify-content: center;
  align-items: center;
}

/* Spacing utilities */
.mt-1 { margin-top: 0.5rem; }
.mt-2 { margin-top: 1rem; }
.mt-3 { margin-top: 1.5rem; }

Quick Reference

Rule	Convention
Indentation	2 spaces
Case	Lowercase
Quotes	Single (') except @charset
Semicolons	Required after every declaration
Units	Omit after 0
Leading zeros	Always include (0.8em)
Hex colors	3-char when possible, lowercase
ID selectors	Avoid
Type selectors	Don't qualify classes
!important	Avoid
Property order	Alphabetical (optional)
Line breaks	New line per selector/declaration
Rule separation	Blank line between rules
Comments	Section comments for organization

Common Patterns

Button Component

.button {
  background-color: #007bff;
  border: 0;
  border-radius: 4px;
  color: #fff;
  cursor: pointer;
  display: inline-block;
  font-size: 1rem;
  padding: 0.5rem 1rem;
  text-align: center;
  text-decoration: none;
}

.button:hover {
  background-color: #0056b3;
}

.button-secondary {
  background-color: #6c757d;
}

.button-large {
  font-size: 1.25rem;
  padding: 0.75rem 1.5rem;
}

Card Component

.card {
  background: #fff;
  border: 1px solid #ddd;
  border-radius: 8px;
  padding: 1.5rem;
}

.card-header {
  border-bottom: 1px solid #ddd;
  margin-bottom: 1rem;
  padding-bottom: 0.5rem;
}

.card-title {
  font-size: 1.5rem;
  margin: 0;
}

.card-body {
  line-height: 1.6;
}

Grid System

.grid {
  display: grid;
  gap: 1rem;
  grid-template-columns: repeat(12, 1fr);
}

.col-4 {
  grid-column: span 4;
}

.col-6 {
  grid-column: span 6;
}

.col-12 {
  grid-column: span 12;
}

Additional Resources

• references/css-detailed.md - Advanced CSS patterns, methodologies, and best practices
• W3C CSS Validator
• Google HTML/CSS Style Guide

Summary

Write CSS that is:

• Valid: Passes W3C validation
• Semantic: Meaningful class names
• Maintainable: Low specificity, avoid IDs
• Consistent: Follow formatting rules
• Efficient: Use shorthand, avoid repetition
• Readable: Proper spacing and organization
• Lowercase: All selectors and properties
• Quoted: Single quotes (except @charset)