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 GSS
files. Tools are free to obfuscate, minify, and compile as
long as the general code quality is maintained.




Omit the protocol portion (http:, https:) from URLs pointing to images and other
media files, style sheets, and scripts unless the respective
files are not available over both protocols.

Omitting the protocol—which makes the URL
relative—prevents mixed content issues and results in
minor file size savings.

<!-- Not recommended -->
<script src=""></script>
<!-- Recommended -->
<script src="//"></script>
/* Not recommended */
.example {
  background: url(;
/* Recommended */
.example {
  background: url(//;




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

.example {
  color: blue;



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).

<!-- Not recommended -->
<A HREF="/">Home</A>
<!-- Recommended -->
<img  data-original="google.png" alt="Google">
/* Not recommended */
color: #E5E5E5;
/* Recommended */
color: #e5e5e5;



Trailing white spaces are unnecessary and can complicate

<!-- Not recommended -->
<!-- Recommended -->
<p>Yes please.



使用 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



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

(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.)



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

Append a contact (username or mailing list) in parentheses
as with the format TODO(contact).

Append action items after a colon as in TODO: action

{# TODO(john.doe): revisit centering #}
<!-- TODO: remove optional tags -->



使用 HTML5。

HTML5 (HTML syntax) is preferred for all HTML documents: <!DOCTYPE html>.

(It’s recommended to use HTML, as text/html. Do not use
XHTML. XHTML, as application/xhtml+xml,
lacks both browser and infrastructure support and offers
less room for optimization than HTML.)

Although fine with HTML, do not close void elements, i.e. write <br>, not <br />.



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
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.

<!-- Not recommended -->
<article>This is only a test.
<!-- Recommended -->
<!DOCTYPE html>
<meta charset="utf-8">
<article>This is only a test.</article>



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.

<!-- Not recommended -->
<div onclick="goToRecommendations();">All recommendations</div>
<!-- Recommended -->
<a href="recommendations/">All recommendations</a>



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="".)

<!-- Not recommended -->
<img  data-original="spreadsheet.png">
<!-- Recommended -->
<img  data-original="spreadsheet.png" alt="Spreadsheet screenshot.">



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.

<!-- 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>
<!-- 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!



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

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

<!-- Not recommended -->
The currency symbol for the Euro is &ldquo;&eur;&rdquo;.
<!-- Recommended -->
The currency symbol for the Euro is “€”.



For file size optimization and scannability purposes,
consider omitting optional tags.
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.)

<!-- Not recommended -->
<!DOCTYPE html>
    <title>Spending money, spending bytes</title>
<!-- Recommended -->
<!DOCTYPE html>
<title>Saving money, saving bytes</title>


省去style sheets和scripts上的type属性。

Do not use type attributes for style sheets
(unless not using CSS) and scripts (unless not using

Specifying type attributes in these contexts is
not necessary as HTML5 implies text/css and text/javascript as defaults. This can be safely done even for older browsers.

<!-- Not recommended -->
<link rel="stylesheet" href="//"
<!-- Recommended -->
<link rel="stylesheet" href="//">
<!-- Not recommended -->
<script src="//"
<!-- Recommended -->
<script src="//"></script>



每个代码块,list, table元素都另起新行,缩进它们的子元素。

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

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.)

  <p><em>Space</em>, the final frontier.</p>
      <th scope="col">Income
      <th scope="col">Taxes
      <td>$ 5.00
      <td>$ 4.50



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

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




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

Use tools such as the W3C
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.



Instead of presentational or cryptic names, always use ID
and 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.

/* Not recommended: meaningless */
#yee-1901 {}

/* Not recommended: presentational */
.button-green {}
.clear {}
/* Recommended: specific */
#gallery {}
#login {}
.video {}

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



Try to convey what an ID or class is about while being as
brief as possible.

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

/* Not recommended */
#navigation {}
.atr {}
/* Recommended */
#nav {}
.author {}



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

Avoiding unnecessary ancestor selectors is useful for performance

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



CSS offers a variety of 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

/* 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;



Do not use units after 0 values unless they are

margin: 0;
padding: 0;



Do not use put 0s in front of values or lengths
between -1 and 1.

font-size: .8em;



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

/* Not recommended */
color: #eebbcc;
/* Recommended */
color: #ebc;



In large projects as well as for code that gets embedded in
other projects or on external sites use prefixes (as
namespaces) for ID and 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

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



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.

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

/* Not recommended: uses underscore instead of hyphen */
.error_status {}
/* Recommended */
#video-id {}
.ads-sample {}


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.




Put declarations in alphabetical order in order to achieve
consistent code in a way that is easy to remember and

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).

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



Indent all block
, that is rules within rules as well as declarations, so to
reflect hierarchy and improve understanding.

@media screen, projection {

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




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

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



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

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



Always use a single space between the last selector and the opening
brace that begins the declaration

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

/* Not recommended: missing space */
  margin-top: 1em;

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



Always start a new line for each selector and declaration.

/* Not recommended */
a:focus, a:active {
  position: relative; top: 1px;
/* Recommended */
h3 {
  font-weight: normal;
  line-height: 1.2;



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

html {
  background: #fff;

body {
  margin: auto;
  width: 50%;



Use single ('') rather than double ("")
quotation marks for attribute selectors or 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

/* Not recommended */
@import url("//");

html {
  font-family: "open sans", arial, sans-serif;
/* Recommended */
@import url(//;

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




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

/* Header */

#adw-header {}

/* Footer */

#adw-footer {}

/* Gallery */

.adw-gallery {}



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.

版本 2.23



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