styleguide/htmlcssguide.md

<div class="numbered"></div>

# Google HTML/CSS Style Guide

<!-- mdformat off -->
* TOC
{:toc}
<!-- mdformat on -->

<div class="numbered"></div>

<a id="Background"></a>

## Background

This document defines formatting and style rules for HTML and CSS. It aims at
improving collaboration, code quality, and enabling supporting infrastructure.
It applies to raw, working files that use HTML and CSS, including Sass and GSS
files. Tools are free to obfuscate, minify, and compile as long as the general
code quality is maintained.

<div class="numbered"></div>

<a id="General"></a>

## General

<div class="numbered"></div>

<a id="General_Style_Rules"></a>

### General Style Rules

<div class="numbered"></div>

<a id="Protocol"></a>

#### Protocol

Use HTTPS for embedded resources where possible.

Always use HTTPS (`https:`) for images and other media files, style sheets, and
scripts, unless the respective files are not available over HTTPS.

<div class='badcode'></div>

```html
<!-- Not recommended: omits the protocol -->
<script src="//ajax.googleapis.com/ajax/libs/jquery/3.4.0/jquery.min.js"></script>

<!-- Not recommended: uses HTTP -->
<script src="http://ajax.googleapis.com/ajax/libs/jquery/3.4.0/jquery.min.js"></script>
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.0/jquery.min.js"></script>
```

<div class='badcode'></div>

```css
/* Not recommended: omits the protocol */
@import '//fonts.googleapis.com/css?family=Open+Sans';

/* Not recommended: uses HTTP */
@import 'http://fonts.googleapis.com/css?family=Open+Sans';
```

<div class='goodcode'></div>

```css
/* Recommended */
@import 'https://fonts.googleapis.com/css?family=Open+Sans';
```

<div class="numbered"></div>

<a id="General_Formatting_Rules"></a>

### General Formatting Rules

<div class="numbered"></div>

<a id="Indentation"></a>

#### Indentation

Indent by 2 spaces at a time.

Don’t use tabs or mix tabs and spaces for indentation.

<div class='goodcode'></div>

```html
<ul>
  <li>Fantastic
  <li>Great
</ul>
```

<div class='goodcode'></div>

```css
.example {
  color: blue;
}
```

<div class="numbered"></div>

<a id="Capitalization"></a>

#### Capitalization

Use only lowercase.

All code has to be lowercase: This applies to HTML element names, attributes,
attribute values (unless `text/CDATA`), CSS selectors, properties, and property
values (with the exception of strings).

<div class='badcode'></div>

```html
<!-- Not recommended -->
<A HREF="/">Home</A>
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<img src="google.png" alt="Google">
```

<div class='badcode'></div>

```css
/* Not recommended */
color: #E5E5E5;
```

<div class='goodcode'></div>

```css
/* Recommended */
color: #e5e5e5;
```

<div class="numbered"></div>

<a id="Trailing_Whitespace"></a>

#### Trailing Whitespace

Remove trailing white spaces.

Trailing white spaces are unnecessary and can complicate diffs.

<div class='badcode'></div>

```html
<!-- Not recommended -->
<p>What?_
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<p>Yes please.
```

<div class="numbered"></div>

<a id="General_Meta_Rules"></a>

### General Meta Rules

<div class="numbered"></div>

<a id="Encoding"></a>

#### Encoding

Use UTF-8 (no BOM).

Make sure your editor uses UTF-8 as character encoding, without a byte order
mark.

Specify the encoding in HTML templates and documents via
`<meta charset="utf-8">`. Do not specify the encoding of style sheets as these
assume UTF-8.

(More on encodings and when and how to specify them can be found in
[Handling character encodings in HTML and CSS](https://www.w3.org/International/tutorials/tutorial-char-enc/).)

<div class="numbered"></div>

<a id="Comments"></a>

#### Comments

Explain code as needed, where possible.

Use comments to explain code: What does it cover, what purpose does it serve,
why is respective solution used or preferred?

(This item is optional as it is not deemed a realistic expectation to always
demand fully documented code. Mileage may vary heavily for HTML and CSS code and
depends on the project’s complexity.)

<div class="numbered"></div>

<a id="Action_Items"></a>

#### Action Items

Mark todos and action items with `TODO`.

Highlight todos by using the keyword `TODO` only, not other common formats like
`@@`.

Append action items after a colon as in `TODO: action item`.

<div class='goodcode'></div>

```django
{# TODO: Revisit centering. #}
<center>Test</center>
```

<div class='goodcode'></div>

```html
<!-- TODO: Remove optional tags. -->
<ul>
  <li>Apples</li>
  <li>Oranges</li>
</ul>
```

<div class="numbered"></div>

<a id="HTML"></a>

## HTML

<div class="numbered"></div>

<a id="HTML_Style_Rules"></a>

### HTML Style Rules

<div class="numbered"></div>

<a id="Document_Type"></a>

#### Document Type

Use `<!doctype html>`.

Always put your HTML in
[no-quirks mode](https://developer.mozilla.org/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode)
by including `<!doctype html>` at the beginning of the document.

A document without a doctype is rendered in “quirks mode”, and one with a
different doctype may be rendered in “limited-quirks mode”. These modes don’t
follow the widely-understood, widely-documented behavior for various core HTML
and CSS constructs, and are likely to cause subtle failures and
incompatibilities especially when reusing code that expects no-quirks mode.

<div class="numbered"></div>

<a id="HTML_Validity"></a>

#### HTML Validity

Use valid HTML where possible.

Use valid HTML code unless that is not possible due to otherwise unattainable
performance goals regarding file size.

Use tools such as the [W3C HTML validator](https://validator.w3.org/nu/) to
test.

Using valid HTML is a measurable baseline quality attribute that contributes to
learning about technical requirements and constraints, and that ensures proper
HTML usage.

<div class='badcode'></div>

```html
<!-- Not recommended -->
<title>Test</title>
<article>This is only a test.
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<!doctype html>
<meta charset="utf-8">
<title>Test</title>
<article>This is only a test.</article>
```

<div class="numbered"></div>

<a id="Semantics"></a>

#### Semantics

Use HTML according to its purpose.

Use elements (sometimes incorrectly called “tags”) for what they have been
created for. For example, use heading elements for headings, `p` elements for
paragraphs, `a` elements for anchors, etc.

Using HTML according to its purpose is important for accessibility, reuse, and
code efficiency reasons.

<div class='badcode'></div>

```html
<!-- Not recommended -->
<div onclick="goToRecommendations();">All recommendations</div>
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<a href="recommendations/">All recommendations</a>
```

<div class="numbered"></div>

<a id="Multimedia_Fallback"></a>

#### Multimedia Fallback

Provide alternative contents for multimedia.

For multimedia, such as images, videos, animated objects via `canvas`, make sure
to offer alternative access. For images that means use of meaningful alternative
text (`alt`) and for video and audio transcripts and captions, if available.

Providing alternative contents is important for accessibility reasons: A blind
user has few cues to tell what an image is about without `@alt`, and other users
may have no way of understanding what video or audio contents are about either.

(For images whose `alt` attributes would introduce redundancy, and for images
whose purpose is purely decorative which you cannot immediately use CSS for, use
no alternative text, as in `alt=""`.)

<div class='badcode'></div>

```html
<!-- Not recommended -->
<img src="spreadsheet.png">
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<img src="spreadsheet.png" alt="Spreadsheet screenshot.">
```

<div class="numbered"></div>

<a id="Separation_of_Concerns"></a>

#### Separation of Concerns

Separate structure from presentation from behavior.

Strictly keep structure (markup), presentation (styling), and behavior
(scripting) apart, and try to keep the interaction between the three to an
absolute minimum.

That is, make sure documents and templates contain only HTML and HTML that is
solely serving structural purposes. Move everything presentational into style
sheets, and everything behavioral into scripts.

In addition, keep the contact area as small as possible by linking as few style
sheets and scripts as possible from documents and templates.

Separating structure from presentation from behavior is important for
maintenance reasons. It is always more expensive to change HTML documents and
templates than it is to update style sheets and scripts.

<div class='badcode'></div>

```html
<!-- Not recommended -->
<!doctype html>
<title>HTML sucks</title>
<link rel="stylesheet" href="base.css" media="screen">
<link rel="stylesheet" href="grid.css" media="screen">
<link rel="stylesheet" href="print.css" media="print">
<h1 style="font-size: 1em;">HTML sucks</h1>
<p>I’ve read about this on a few sites but now I’m sure:
  <u>HTML is stupid!!1</u>
<center>I can’t believe there’s no way to control the styling of
  my website without doing everything all over again!</center>
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<!doctype html>
<title>My first CSS-only redesign</title>
<link rel="stylesheet" href="default.css">
<h1>My first CSS-only redesign</h1>
<p>I’ve read about this on a few sites but today I’m actually
  doing it: separating concerns and avoiding anything in the HTML of
  my website that is presentational.
<p>It’s awesome!
```

<div class="numbered"></div>

<a id="Entity_References"></a>

#### Entity References

Do not use entity references.

There is no need to use entity references like `&mdash;`, `&rdquo;`, or
`&#x263a;`, assuming the same encoding (UTF-8) is used for files and editors as
well as among teams.

The only exceptions apply to characters with special meaning in HTML (like `<`
and `&`) as well as control or “invisible” characters (like no-break spaces).

<div class='badcode'></div>

```html
<!-- Not recommended -->
The currency symbol for the Euro is &ldquo;&eur;&rdquo;.
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
The currency symbol for the Euro is “€”.
```

<div class="numbered"></div>

<a id="Optional_Tags"></a>

#### Optional Tags

Omit optional tags (optional).

For file size optimization and scannability purposes, consider omitting optional
tags. The
[HTML5 specification](https://html.spec.whatwg.org/multipage/syntax.html#syntax-tag-omission)
defines what tags can be omitted.

(This approach may require a grace period to be established as a wider guideline
as it’s significantly different from what web developers are typically taught.
For consistency and simplicity reasons it’s best served omitting all optional
tags, not just a selection.)

<div class='badcode'></div>

```html
<!-- Not recommended -->
<!doctype html>
<html>
  <head>
    <title>Spending money, spending bytes</title>
  </head>
  <body>
    <p>Sic.</p>
  </body>
</html>
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<!doctype html>
<title>Saving money, saving bytes</title>
<p>Qed.
```

<div class="numbered"></div>

<a id="type_Attributes"></a>

#### `type` Attributes

Omit `type` attributes for style sheets and scripts.

Do not use `type` attributes for style sheets (unless not using CSS) and scripts
(unless not using JavaScript, or when loading a JavaScript module with
`type="module"`).

Specifying `type` attributes in these contexts is not necessary as HTML5 implies
[`text/css`](https://html.spec.whatwg.org/multipage/obsolete.html#attr-style-type)
and
[`text/javascript`](https://html.spec.whatwg.org/multipage/scripting.html#attr-script-type)
as defaults. This can be safely done even for older browsers.

<div class='badcode'></div>

```html
<!-- Not recommended -->
<link rel="stylesheet" href="https://www.google.com/css/maia.css"
    type="text/css">
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<link rel="stylesheet" href="https://www.google.com/css/maia.css">
```

<div class='badcode'></div>

```html
<!-- Not recommended -->
<script src="https://www.google.com/js/gweb/analytics/autotrack.js"
    type="text/javascript"></script>
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<script src="https://www.google.com/js/gweb/analytics/autotrack.js"></script>
```

[JavaScript modules](https://html.spec.whatwg.org/multipage/scripting.html#attr-script-type)
must use `type="module"` when loading via `<script>`.

<div class='goodcode'></div>

```html
<!-- Recommended -->
<script src="https://example.com/js/app.js" type="module"></script>
```

<div class="numbered"></div>

<a id="id_Attributes"></a>

#### `id` Attributes

Avoid unnecessary `id` attributes.

Prefer `class` attributes for styling and `data` attributes for scripting.

Where `id` attributes are strictly required, always include a hyphen in the
value to ensure it does not match the JavaScript identifier syntax, e.g. use
`user-profile` rather than just `profile` or `userProfile`.

When an element has an `id` attribute, browsers will make that available as a
[named property on the global `window` prototype](https://html.spec.whatwg.org/multipage/window-object.html#named-access-on-the-window-object),
which may cause unexpected behavior. While `id` attribute values containing a
hyphen are still available as property names, these cannot be referenced as
global JavaScript variables.

<div class='badcode'></div>

```html
<!-- Not recommended: `window.userProfile` will resolve to reference the <div> node -->
<div id="userProfile"></div>
```

<div class='goodcode'></div>

```html
<!-- Recommended: `id` attribute is required and its value includes a hyphen -->
<div aria-describedby="user-profile">
  …
  <div id="user-profile"></div>
  …
</div>
```

<div class="numbered"></div>

<a id="HTML_Formatting_Rules"></a>

### HTML Formatting Rules

<div class="numbered"></div>

<a id="General_Formatting"></a>

#### General Formatting

Use a new line for every block, list, or table element, and indent every such
child element.

Independent of the styling of an element (as CSS allows elements to assume a
different role per `display` property), put every block, list, or table element
on a new line.

Also, indent them if they are child elements of a block, list, or table element.

(If you run into issues around whitespace between list items it’s acceptable to
put all `li` elements in one line. A linter is encouraged to throw a warning
instead of an error.)

<div class='goodcode'></div>

```html
<blockquote>
  <p><em>Space</em>, the final frontier.</p>
</blockquote>
```

<div class='goodcode'></div>

```html
<ul>
  <li>Moe
  <li>Larry
  <li>Curly
</ul>
```

<div class='goodcode'></div>

```html
<table>
  <thead>
    <tr>
      <th scope="col">Income
      <th scope="col">Taxes
  <tbody>
    <tr>
      <td>$ 5.00
      <td>$ 4.50
</table>
```

<div class="numbered"></div>

<a id="HTML_Line-Wrapping"></a>

#### HTML Line-Wrapping

Break long lines (optional).

While there is no column limit recommendation for HTML, you may consider
wrapping long lines if it significantly improves readability.

When line-wrapping, each continuation line should be indented to distinguish
wrapped attributes from child elements. Lines should be wrapped consistently
within a project, ideally enforced by automated code formatting tools.

<div class='goodcode'></div>

```html
<button
  mat-icon-button
  color="primary"
  class="menu-button"
  (click)="openMenu()"
>
  <mat-icon>menu</mat-icon>
</button>
```

<div class='goodcode'></div>

```html
<button mat-icon-button color="primary" class="menu-button"
    (click)="openMenu()">
  <mat-icon>menu</mat-icon>
</button>
```

<div class='goodcode'></div>

```html
<button
    mat-icon-button
    color="primary"
    class="menu-button"
    (click)="openMenu()">
  <mat-icon>menu</mat-icon>
</button>
```

<div class='goodcode'></div>

```html
<button mat-icon-button
        color="primary"
        class="menu-button"
        (click)="openMenu()">
  <mat-icon>menu</mat-icon>
</button>
```

<div class="numbered"></div>

<a id="HTML_Quotation_Marks"></a>

#### HTML Quotation Marks

When quoting attribute values, use double quotation marks.

Use double (`""`) rather than single quotation marks (`''`) around attribute
values.

<div class='badcode'></div>

```html
<!-- Not recommended -->
<a class='maia-button maia-button-secondary'>Sign in</a>
```

<div class='goodcode'></div>

```html
<!-- Recommended -->
<a class="maia-button maia-button-secondary">Sign in</a>
```

<div class="numbered"></div>

<a id="CSS"></a>

## CSS

<div class="numbered"></div>

<a id="CSS_Style_Rules"></a>

### CSS Style Rules

<div class="numbered"></div>

<a id="CSS_Validity"></a>

#### CSS Validity

Use valid CSS where possible.

Unless dealing with CSS validator bugs or requiring proprietary syntax, use
valid CSS code.

Use tools such as the [W3C CSS validator](https://jigsaw.w3.org/css-validator/)
to test.

Using valid CSS is a measurable baseline quality attribute that allows to spot
CSS code that may not have any effect and can be removed, and that ensures
proper CSS usage.

<div class="numbered"></div>

<a id="ID_and_Class_Naming"></a>

#### Class Naming

Use meaningful or generic class names.

Instead of presentational or cryptic names, always use class names that reflect
the purpose of the element in question, or that are otherwise generic.

Names that are specific and reflect the purpose of the element should be
preferred as these are most understandable and the least likely to change.

Generic names are simply a fallback for elements that have no particular or no
meaning different from their siblings. They are typically needed as “helpers.”

Using functional or generic names reduces the probability of unnecessary
document or template changes.

<div class='badcode'></div>

```css
/* Not recommended: meaningless */
.yee-1901 {}

/* Not recommended: presentational */
.button-green {}
.clear {}
```

<div class='goodcode'></div>

```css
/* Recommended: specific */
.gallery {}
.login {}
.video {}

/* Recommended: generic */
.aux {}
.alt {}
```

<div class="numbered"></div>

<a id="ID_and_Class_Name_Style"></a>

#### Class Name Style

Use class names that are as short as possible but as long as necessary.

Try to convey what a class is about while being as brief as possible.

Using class names this way contributes to acceptable levels of understandability
and code efficiency.

<div class='badcode'></div>

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

<div class='goodcode'></div>

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

<div class="numbered"></div>

<a id="ID_and_Class_Name_Delimiters"></a>

#### Class Name Delimiters

Separate words in class names by a hyphen.

Do not concatenate words and abbreviations in selectors by any characters
(including none at all) other than hyphens, in order to improve understanding
and scannability.

<div class='badcode'></div>

```css
/* Not recommended: does not separate the words “demo” and “image” */
.demoimage {}

/* Not recommended: uses underscore instead of hyphen */
.error_status {}
```

<div class='goodcode'></div>

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

<div class="numbered"></div>

<a id="Prefixes"></a>

#### Prefixes

Prefix selectors with an application-specific prefix (optional).

In large projects as well as for code that gets embedded in other projects or on
external sites use prefixes (as namespaces) for class names. Use short, unique
identifiers followed by a dash.

Using namespaces helps preventing naming conflicts and can make maintenance
easier, for example in search and replace operations.

<div class='goodcode'></div>

```css
.adw-help {} /* AdWords */
.maia-note {} /* Maia */
```

<div class="numbered"></div>

<a id="Type_Selectors"></a>

#### Type Selectors

Avoid qualifying class names with type selectors.

Unless necessary (for example with helper classes), do not use element names in
conjunction with classes.

Avoiding unnecessary ancestor selectors is useful for
[performance reasons](https://www.stevesouders.com/blog/2009/06/18/simplifying-css-selectors/).

<div class='badcode'></div>

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

<div class='goodcode'></div>

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

<div class="numbered"></div>

<a id="ID_Selectors"></a>

#### ID Selectors

Avoid ID selectors.

ID attributes are expected to be unique across an entire page, which is
difficult to guarantee when a page contains many components worked on by many
different engineers. Class selectors should be preferred in all situations.

<div class='badcode'></div>

```css
/* Not recommended */
#example {}
```

<div class='goodcode'></div>

```css
/* Recommended */
.example {}
```

<div class="numbered"></div>

<a id="Shorthand_Properties"></a>

#### Shorthand Properties

Use shorthand properties where possible.

CSS offers a variety of
[shorthand](https://www.w3.org/TR/CSS21/about.html#shorthand) properties (like
`font`) that should be used whenever possible, even in cases where only one
value is explicitly set.

Using shorthand properties is useful for code efficiency and understandability.

<div class='badcode'></div>

```css
/* 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;
```

<div class='goodcode'></div>

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

<div class="numbered"></div>

<a id="0_and_Units"></a>

#### 0 and Units

Omit unit specification after “0” values, unless required.

Do not use units after `0` values unless they are required.

<div class='goodcode'></div>

```css
flex: 0px; /* This flex-basis component requires a unit. */
flex: 1 1 0px; /* Not ambiguous without the unit, but needed in IE11. */
margin: 0;
padding: 0;
```

<div class="numbered"></div>

<a id="Leading_0s"></a>

#### Leading 0s

Always include leading “0”s in values.

Put `0`s in front of values or lengths between -1 and 1.

<div class='goodcode'></div>

```css
font-size: 0.8em;
```

<div class="numbered"></div>

<a id="Hexadecimal_Notation"></a>

#### Hexadecimal Notation

Use 3 character hexadecimal notation where possible.

For color values that permit it, 3 character hexadecimal notation is shorter and
more succinct.

<div class='badcode'></div>

```css
/* Not recommended */
color: #eebbcc;
```

<div class='goodcode'></div>

```css
/* Recommended */
color: #ebc;
```

<div class="numbered"></div>

<a id="Important_Declarations"></a>

#### Important Declarations

Avoid using `!important` declarations.

These declarations break the natural cascade of CSS and make it difficult to
reason about and compose styles. Use [selector specificity] to override
properties instead.

[selector specificity]: https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity

<div class='badcode'></div>

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

<div class='goodcode'></div>

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

<div class="numbered"></div>

<a id="Hacks"></a>

#### Hacks

Avoid user agent detection as well as CSS “hacks”—try a different approach
first.

It’s tempting to address styling differences over user agent detection or
special CSS filters, workarounds, and hacks. Both approaches should be
considered last resort in order to achieve and maintain an efficient and
manageable code base. Put another way, giving detection and hacks a free pass
will hurt projects in the long run as projects tend to take the way of least
resistance. That is, allowing and making it easy to use detection and hacks
means using detection and hacks more frequently—and more frequently is too
frequently.

<div class="numbered"></div>

<a id="CSS_Formatting_Rules"></a>

### CSS Formatting Rules

<div class="numbered"></div>

<a id="Declaration_Order"></a>

#### Declaration Order

Alphabetize declarations (optional).

Sort declarations consistently within a project. In the absence of tooling to
automate and enforce a consistent sort order, consider putting declarations in
alphabetical order in order to achieve consistent code in a way that is easy to
learn, remember, and manually maintain.

Ignore vendor-specific prefixes for sorting purposes. However, multiple
vendor-specific prefixes for a certain CSS property should be kept sorted (e.g.
-moz prefix comes before -webkit).

<div class='goodcode'></div>

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

<div class="numbered"></div>

<a id="Block_Content_Indentation"></a>

#### Block Content Indentation

Indent all block content.

Indent all [block content](https://www.w3.org/TR/CSS21/syndata.html#block), that
is rules within rules as well as declarations, so to reflect hierarchy and
improve understanding.

<div class='goodcode'></div>

```css
@media screen, projection {

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

}
```

<div class="numbered"></div>

<a id="Declaration_Stops"></a>

#### Declaration Stops

Use a semicolon after every declaration.

End every declaration with a semicolon for consistency and extensibility
reasons.

<div class='badcode'></div>

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

<div class='goodcode'></div>

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

<div class="numbered"></div>

<a id="Property_Name_Stops"></a>

#### Property Name Stops

Use a space after a property name’s colon.

Always use a single space between property and value (but no space between
property and colon) for consistency reasons.

<div class='badcode'></div>

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

<div class='goodcode'></div>

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

<div class="numbered"></div>

<a id="Declaration_Block_Separation"></a>

#### Declaration Block Separation

Use a space between the last selector and the declaration block.

Always use a single space between the last selector and the opening brace that
begins the
[declaration block](https://www.w3.org/TR/CSS21/syndata.html#rule-sets).

The opening brace should be on the same line as the last selector in a given
rule.

<div class='badcode'></div>

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

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

<div class='goodcode'></div>

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

<div class="numbered"></div>

<a id="Selector_and_Declaration_Separation"></a>

#### Selector and Declaration Separation

Separate selectors and declarations by new lines.

Always start a new line for each selector and declaration.

<div class='badcode'></div>

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

<div class='goodcode'></div>

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

<div class="numbered"></div>

<a id="Rule_Separation"></a>

#### Rule Separation

Separate rules by new lines.

Always put a blank line (two line breaks) between rules.

<div class='goodcode'></div>

```css
html {
  background: #fff;
}

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

<div class="numbered"></div>

<a id="CSS_Quotation_Marks"></a>

#### CSS Quotation Marks

Use single (`''`) rather than double (`""`) quotation marks for attribute
selectors and property values.

Do not use quotation marks in URI values (`url()`).

Exception: If you do need to use the `@charset` rule, use double quotation
marks—[single quotation marks are not permitted](https://www.w3.org/TR/CSS21/syndata.html#charset).

<div class='badcode'></div>

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

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

<div class='goodcode'></div>

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

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

<div class="numbered"></div>

<a id="CSS_Meta_Rules"></a>

### CSS Meta Rules

<div class="numbered"></div>

<a id="Section_Comments"></a>

#### Section Comments

Group sections by a section comment (optional).

If possible, group style sheet sections together by using comments. Separate
sections with new lines.

<div class='goodcode'></div>

```css
/* Header */

.adw-header {}

/* Footer */

.adw-footer {}

/* Gallery */

.adw-gallery {}
```

<a id="Parting_Words"></a>

## Parting Words

*Be consistent.*

If you’re editing code, take a few minutes to look at the code around you and
determine its style. If they use spaces around all their arithmetic operators,
you should too. If their comments have little boxes of hash marks around them,
make your comments have little boxes of hash marks around them too.

The point of having style guidelines is to have a common vocabulary of coding so
people can concentrate on what you’re saying rather than on how you’re saying
it. We present global style rules here so people know the vocabulary, but local
style is also important. If code you add to a file looks drastically different
from the existing code around it, it throws readers out of their rhythm when
they go to read it. Avoid this.