This document defines formatting and style rules for HTML and CSS. It aims atimproving 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 codequality is maintained.
Use HTTPS for embedded resources where possible.
Always use HTTPS ( https:) for images and other mediafiles, style sheets, and scripts, unless the respective files are not availableover HTTPS.
<!-- 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>
<!-- Recommended --><script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.0/jquery.min.js"></script>
/* 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';
/* Recommended */@import 'https://fonts.googleapis.com/css?family=Open+Sans';
Indent by 2 spaces at a time.
Don’t use tabs or mix tabs and spaces for indentation.
<ul> <li>Fantastic <li>Great</ul>
.example { color: blue;} 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 propertyvalues (with the exception of strings).
<!-- Not recommended --><A HREF="/">Home</A>
<!-- Recommended --><img src="google.png" alt="Google">
/* Not recommended */color: #E5E5E5;
/* Recommended */color: #e5e5e5;
Remove trailing white spaces.
Trailing white spaces are unnecessary and can complicate diffs.
<!-- Not recommended --><p>What?_
<!-- Recommended --><p>Yes please.
Use UTF-8 (no BOM).
Make sure your editor uses UTF-8 as character encoding, without a byte ordermark.
Specify the encoding in HTML templates and documents via <metacharset="utf-8">. Do not specify the encoding of style sheets as these assumeUTF-8.
(More on encodings and when and how to specify them can be found in Handlingcharacter encodings in HTML and CSS.)
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 alwaysdemand fully documented code. Mileage may vary heavily for HTML and CSS code anddepends on the project’s complexity.)
Mark todos and action items with TODO.
Highlight todos by using the keyword TODOonly, 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 item.
{# TODO(john.doe): revisit centering #}<center>Test</center> <!-- TODO: remove optional tags --><ul> <li>Apples</li> <li>Oranges</li></ul>
Use 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 browserand 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 where possible.
Use valid HTML code unless that is not possible due to otherwise unattainableperformance goals regarding file size.
Use tools such as the W3C HTML validatorto test.
Using valid HTML is a measurable baseline quality attribute that contributes tolearning about technical requirements and constraints, and that ensures properHTML usage.
<!-- Not recommended --><title>Test</title><article>This is only a test.
<!-- Recommended --><!DOCTYPE html><meta charset="utf-8"><title>Test</title><article>This is only a test.</article>
Use HTML according to its purpose.
Use elements (sometimes incorrectly called “tags”) for what they have beencreated for. For example, use heading elements for headings, pelements forparagraphs, aelements for anchors, etc.
Using HTML according to its purpose is important for accessibility, reuse, andcode efficiency reasons.
<!-- Not recommended --><div onclick="goToRecommendations();">All recommendations</div>
<!-- Recommended --><a href="recommendations/">All recommendations</a>
Provide alternative contents for multimedia.
For multimedia, such as images, videos, animated objects via canvas, make sureto offer alternative access. For images that means use of meaningful alternativetext ( alt) and for video and audio transcripts and captions, if available.
Providing alternative contents is important for accessibility reasons: A blinduser has few cues to tell what an image is about without @alt, and other usersmay have no way of understanding what video or audio contents are about either.
(For images whose altattributes would introduce redundancy, and for imageswhose purpose is purely decorative which you cannot immediately use CSS for, useno alternative text, as in alt="".)
<!-- Not recommended --><img src="spreadsheet.png">
<!-- Recommended --><img src="spreadsheet.png" alt="Spreadsheet screenshot.">
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 anabsolute minimum.
That is, make sure documents and templates contain only HTML and HTML that issolely serving structural purposes. Move everything presentational into stylesheets, and everything behavioral into scripts.
In addition, keep the contact area as small as possible by linking as few stylesheets and scripts as possible from documents and templates.
Separating structure from presentation from behavior is important formaintenance reasons. It is always more expensive to change HTML documents andtemplates 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!
Do not use entity references.
There is no need to use entity references like —, ”, or ☺, assuming the same encoding (UTF-8) is used for files and editorsas 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).
<!-- Not recommended -->The currency symbol for the Euro is “&eur;”.
<!-- Recommended -->The currency symbol for the Euro is “€”.
Omit optional tags (optional).
For file size optimization and scannability purposes, consider omitting optionaltags. The HTML5 specificationdefines what tags can be omitted.
(This approach may require a grace period to be established as a wider guidelineas it’s significantly different from what web developers are typically taught.For consistency and simplicity reasons it’s best served omitting all optionaltags, not just a selection.)
<!-- Not recommended --><!DOCTYPE html><html> <head> <title>Spending money, spending bytes</title> </head> <body> <p>Sic.</p> </body></html>
<!-- Recommended --><!DOCTYPE html><title>Saving money, saving bytes</title><p>Qed.
typeAttributes Omit typeattributes for style sheets and scripts.
Do not use typeattributes for style sheets (unless not using CSS) and scripts(unless not using JavaScript).
Specifying typeattributes 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="https://www.google.com/css/maia.css" type="text/css">
<!-- Recommended --><link rel="stylesheet" href="https://www.google.com/css/maia.css">
<!-- Not recommended --><script src="https://www.google.com/js/gweb/analytics/autotrack.js" type="text/javascript"></script>
<!-- Recommended --><script src="https://www.google.com/js/gweb/analytics/autotrack.js"></script>
Use a new line for every block, list, or table element, and indent every suchchild element.
Independent of the styling of an element (as CSS allows elements to assume adifferent role per displayproperty), put every block, list, or table elementon 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 toput all lielements in one line. A linter is encouraged to throw a warninginstead of an error.)
<blockquote> <p><em>Space</em>, the final frontier.</p></blockquote>
<ul> <li>Moe <li>Larry <li>Curly</ul>
<table> <thead> <tr> <th scope="col">Income <th scope="col">Taxes <tbody> <tr> <td>$ 5.00 <td>$ 4.50</table>
Break long lines (optional).
While there is no column limit recommendation for HTML, you may considerwrapping long lines if it significantly improves readability.
When line-wrapping, each continuation line should be indented at least 4additional spaces from the original line.
<md-progress-circular md-mode="indeterminate" class="md-accent" ng-show="ctrl.loading" md-diameter="35"></md-progress-circular>
<md-progress-circular md-mode="indeterminate" class="md-accent" ng-show="ctrl.loading" md-diameter="35"></md-progress-circular>
<md-progress-circular md-mode="indeterminate" class="md-accent" ng-show="ctrl.loading" md-diameter="35"></md-progress-circular>
When quoting attributes values, use double quotation marks.
Use double ( "") rather than single quotation marks ( '') around attributevalues.
<!-- Not recommended --><a class='maia-button maia-button-secondary'>Sign in</a>
<!-- Recommended --><a class="maia-button maia-button-secondary">Sign in</a>
Use valid CSS where possible.
Unless dealing with CSS validator bugs or requiring proprietary syntax, usevalid CSS code.
Use tools such as the W3C CSS validatorto test.
Using valid CSS is a measurable baseline quality attribute that allows to spotCSS code that may not have any effect and can be removed, and that ensuresproper CSS usage.
Use meaningful or generic ID and class names.
Instead of presentational or cryptic names, always use ID and class names thatreflect the purpose of the element in question, or that are otherwise generic.
Names that are specific and reflect the purpose of the element should bepreferred as these are most understandable and the least likely to change.
Generic names are simply a fallback for elements that have no particular or nomeaning different from their siblings. They are typically needed as “helpers.”
Using functional or generic names reduces the probability of unnecessarydocument or template changes.
/* Not recommended: meaningless */#yee-1901 {}/* Not recommended: presentational */.button-green {}.clear {} /* Recommended: specific */#gallery {}#login {}.video {}/* Recommended: generic */.aux {}.alt {} Use ID and class names that are as short as possible but as long as necessary.
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 ofunderstandability and code efficiency.
/* Not recommended */#navigation {}.atr {} /* Recommended */#nav {}.author {} Avoid qualifying ID and class names with type selectors.
Unless necessary (for example with helper classes), do not use element names inconjunction with IDs or classes.
Avoiding unnecessary ancestor selectors is useful for performance reasons.
/* Not recommended */ul#example {}div.error {} /* Recommended */#example {}.error {} Use shorthand properties where possible.
CSS offers a variety of shorthandproperties (like font) that should be used whenever possible, even in caseswhere only one value is explicitly set.
Using shorthand properties is useful for code efficiency and understandability.
/* 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;
Omit unit specification after “0” values, unless required.
Do not use units after 0values unless they are required.
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;
Omit leading “0”s in values.
Do not put 0s in front of values or lengths between -1 and 1.
font-size: .8em;
Use 3 character hexadecimal notation where possible.
For color values that permit it, 3 character hexadecimal notation is shorter andmore succinct.
/* Not recommended */color: #eebbcc;
/* Recommended */color: #ebc;
Prefix selectors with an application-specific prefix (optional).
In large projects as well as for code that gets embedded in other projects or onexternal 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 maintenanceeasier, for example in search and replace operations.
.adw-help {} /* AdWords */#maia-note {} /* Maia */ Separate words in ID and 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 understandingand 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 approachfirst.
It’s tempting to address styling differences over user agent detection orspecial CSS filters, workarounds, and hacks. Both approaches should beconsidered last resort in order to achieve and maintain an efficient andmanageable code base. Put another way, giving detection and hacks a free passwill hurt projects in the long run as projects tend to take the way of leastresistance. That is, allowing and making it easy to use detection and hacksmeans using detection and hacks more frequently—and more frequently is toofrequently.
Alphabetize declarations.
Put declarations in alphabetical order in order to achieve consistent code in away that is easy to remember and maintain.
Ignore vendor-specific prefixes for sorting purposes. However, multiplevendor-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 content.
Indent all block content,that is rules within rules as well as declarations, so to reflect hierarchy andimprove understanding.
@media screen, projection { html { background: #fff; color: #444; }} Use a semicolon after every declaration.
End every declaration with a semicolon for consistency and extensibilityreasons.
/* Not recommended */.test { display: block; height: 100px} /* Recommended */.test { display: block; height: 100px;} Use a space after a property name’s colon.
Always use a single space between property and value (but no space betweenproperty and colon) for consistency reasons.
/* Not recommended */h3 { font-weight:bold;} /* Recommended */h3 { font-weight: bold;} Use a space between the last selector and the declaration block.
Always use a single space between the last selector and the opening brace thatbegins the declaration block.
The opening brace should be on the same line as the last selector in a givenrule.
/* Not recommended: missing space */#video{ margin-top: 1em;}/* Not recommended: unnecessary line break */#video{ margin-top: 1em;} /* Recommended */#video { margin-top: 1em;} Separate selectors and declarations by new lines.
Always start a 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;} Separate rules by new lines.
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 attributeselectors and property values.
Do not use quotation marks in URI values ( url()).
Exception: If you do need to use the @charsetrule, use double quotationmarks— single quotation marks are not permitted.
/* 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;} Group sections by a section comment (optional).
If possible, group style sheet sections together by using comments. Separatesections with new lines.
/* Header */#adw-header {}/* Footer */#adw-footer {}/* Gallery */.adw-gallery {} Be consistent.
If you’re editing code, take a few minutes to look at the code around you anddetermine 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 sopeople can concentrate on what you’re saying rather than on how you’re sayingit. We present global style rules here so people know the vocabulary, but localstyle is also important. If code you add to a file looks drastically differentfrom the existing code around it, it throws readers out of their rhythm whenthey go to read it. Avoid this.