1<!DOCTYPE html> 2<html lang="en"> 3<head> 4<meta charset="utf-8"> 5<title>Google HTML/CSS Style Guide</title> 6<link rel="stylesheet" href="javaguide.css"> 7<script src="include/styleguide.js"></script> 8<link rel="shortcut icon" href="https://www.google.com/favicon.ico"> 9<script src="include/jsguide.js"></script> 10</head> 11<body onload="initStyleGuide();"> 12<div id="content"> 13<h1>Google HTML/CSS Style Guide</h1> 14<h2 id="Background">1 Background</h2> 15 16<p></p> 17 18<p>This document defines formatting and style rules for HTML and CSS. It aims at 19improving collaboration, code quality, and enabling supporting infrastructure. 20It applies to raw, working files that use HTML and CSS, including GSS files. 21Tools are free to obfuscate, minify, and compile as long as the general code 22quality is maintained.</p> 23 24<p></p> 25 26<p></p> 27 28<h2 id="General">2 General</h2> 29 30<h3 id="General_Style_Rules">2.1 General Style Rules</h3> 31 32<h4 id="Protocol">2.1.1 Protocol</h4> 33 34<p>Use the HTTPS protocol for embedded resources where possible.</p> 35 36<p>Always use the HTTPS protocol (<code>https:</code>) for images and other media 37files, style sheets, and scripts, unless the respective files are not available 38over HTTPS.</p> 39 40<pre><code class="language-html prettyprint badcode"><!-- Not recommended: omits the protocol --> 41<script src="//ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js"></script> 42 43<!-- Not recommended: uses the HTTP protocol --> 44<script src="http://ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js"></script> 45</code></pre> 46 47<pre><code class="language-html prettyprint"><!-- Recommended --> 48<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js"></script> 49</code></pre> 50 51<pre><code class="language-css prettyprint badcode">/* Not recommended: omits the protocol */ 52@import '//fonts.googleapis.com/css?family=Open+Sans'; 53 54/* Not recommended: uses the HTTP protocol */ 55@import 'http://fonts.googleapis.com/css?family=Open+Sans'; 56</code></pre> 57 58<pre><code class="language-css prettyprint">/* Recommended */ 59@import 'https://fonts.googleapis.com/css?family=Open+Sans'; 60</code></pre> 61 62 63 64<p></p> 65 66<p></p> 67 68<p></p> 69 70<h3 id="General_Formatting_Rules">2.2 General Formatting Rules</h3> 71 72<h4 id="Indentation">2.2.1 Indentation</h4> 73 74<p>Indent by 2 spaces at a time.</p> 75 76<p>Don’t use tabs or mix tabs and spaces for indentation.</p> 77 78<pre><code class="language-html prettyprint"><ul> 79 <li>Fantastic 80 <li>Great 81</ul> 82</code></pre> 83 84<pre><code class="language-css prettyprint">.example { 85 color: blue; 86} 87</code></pre> 88 89<h4 id="Capitalization">2.2.2 Capitalization</h4> 90 91<p>Use only lowercase.</p> 92 93<p>All code has to be lowercase: This applies to HTML element names, attributes, 94attribute values (unless <code>text/CDATA</code>), CSS selectors, properties, and property 95values (with the exception of strings).</p> 96 97<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 98<A HREF="/">Home</A> 99</code></pre> 100 101<pre><code class="language-html prettyprint"><!-- Recommended --> 102<img src="google.png" alt="Google"> 103</code></pre> 104 105<pre><code class="language-css prettyprint badcode">/* Not recommended */ 106color: #E5E5E5; 107</code></pre> 108 109<pre><code class="language-css prettyprint">/* Recommended */ 110color: #e5e5e5; 111</code></pre> 112 113<h4 id="Trailing_Whitespace">2.2.3 Trailing Whitespace</h4> 114 115<p>Remove trailing white spaces.</p> 116 117<p>Trailing white spaces are unnecessary and can complicate diffs.</p> 118 119<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 120<p>What?_ 121</code></pre> 122 123<pre><code class="language-html prettyprint"><!-- Recommended --> 124<p>Yes please. 125</code></pre> 126 127<h3 id="General_Meta_Rules">2.3 General Meta Rules</h3> 128 129<h4 id="Encoding">2.3.1 Encoding</h4> 130 131<p>Use UTF-8 (no BOM).</p> 132 133<p>Make sure your editor uses UTF-8 as character encoding, without a byte order 134mark.</p> 135 136<p>Specify the encoding in HTML templates and documents via <code><meta 137charset="utf-8"></code>. Do not specify the encoding of style sheets as these assume 138UTF-8.</p> 139 140<p>(More on encodings and when and how to specify them can be found in <a href="https://www.w3.org/International/tutorials/tutorial-char-enc/">Handling 141character encodings in HTML and CSS</a>.)</p> 142 143<h4 id="Comments">2.3.2 Comments</h4> 144 145<p>Explain code as needed, where possible.</p> 146 147<p>Use comments to explain code: What does it cover, what purpose does it serve, 148why is respective solution used or preferred?</p> 149 150<p>(This item is optional as it is not deemed a realistic expectation to always 151demand fully documented code. Mileage may vary heavily for HTML and CSS code and 152depends on the project’s complexity.)</p> 153 154<h4 id="Action_Items">2.3.3 Action Items</h4> 155 156<p>Mark todos and action items with <code>TODO</code>.</p> 157 158<p>Highlight todos by using the keyword <code>TODO</code> only, not other common formats like 159<code>@@</code>.</p> 160 161<p>Append a contact (username or mailing list) in parentheses as with the format 162<code>TODO(contact)</code>.</p> 163 164<p>Append action items after a colon as in <code>TODO: action item</code>.</p> 165 166<pre></pre> 167 168<pre><code class="language-django prettyprint external">{# TODO(john.doe): revisit centering #} 169<center>Test</center> 170</code></pre> 171 172<p></p> 173 174<pre><code class="language-html prettyprint"><!-- TODO: remove optional tags --> 175<ul> 176 <li>Apples</li> 177 <li>Oranges</li> 178</ul> 179</code></pre> 180 181<h2 id="HTML">3 HTML</h2> 182 183<h3 id="HTML_Style_Rules">3.1 HTML Style Rules</h3> 184 185<h4 id="Document_Type">3.1.1 Document Type</h4> 186 187<p>Use HTML5.</p> 188 189<p>HTML5 (HTML syntax) is preferred for all HTML documents: <code><!DOCTYPE html></code>.</p> 190 191<p>(It’s recommended to use HTML, as <code>text/html</code>. Do not use XHTML. XHTML, as 192<a href="https://hixie.ch/advocacy/xhtml"><code>application/xhtml+xml</code></a>, lacks both browser 193and infrastructure support and offers less room for optimization than HTML.)</p> 194 195<p>Although fine with HTML, do not close void elements, i.e. write <code><br></code>, not 196<code><br /></code>.</p> 197 198<h4 id="HTML_Validity">3.1.2 HTML Validity</h4> 199 200<p>Use valid HTML where possible.</p> 201 202<p>Use valid HTML code unless that is not possible due to otherwise unattainable 203performance goals regarding file size.</p> 204 205<p> 206 207Use tools such as the <a href="https://validator.w3.org/nu/">W3C HTML validator</a> 208to test. 209</p> 210 211<p>Using valid HTML is a measurable baseline quality attribute that contributes to 212learning about technical requirements and constraints, and that ensures proper 213HTML usage.</p> 214 215<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 216<title>Test</title> 217<article>This is only a test. 218</code></pre> 219 220<pre><code class="language-html prettyprint"><!-- Recommended --> 221<!DOCTYPE html> 222<meta charset="utf-8"> 223<title>Test</title> 224<article>This is only a test.</article> 225</code></pre> 226 227<h4 id="Semantics">3.1.3 Semantics</h4> 228 229<p>Use HTML according to its purpose.</p> 230 231<p>Use elements (sometimes incorrectly called “tags”) for what they have been 232created for. For example, use heading elements for headings, <code>p</code> elements for 233paragraphs, <code>a</code> elements for anchors, etc.</p> 234 235<p>Using HTML according to its purpose is important for accessibility, reuse, and 236code efficiency reasons.</p> 237 238<p></p> 239 240<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 241<div onclick="goToRecommendations();">All recommendations</div> 242</code></pre> 243 244<pre><code class="language-html prettyprint"><!-- Recommended --> 245<a href="recommendations/">All recommendations</a> 246</code></pre> 247 248<h4 id="Multimedia_Fallback">3.1.4 Multimedia Fallback</h4> 249 250<p>Provide alternative contents for multimedia.</p> 251 252<p>For multimedia, such as images, videos, animated objects via <code>canvas</code>, make sure 253to offer alternative access. For images that means use of meaningful alternative 254text (<code>alt</code>) and for video and audio transcripts and captions, if available.</p> 255 256<p>Providing alternative contents is important for accessibility reasons: A blind 257user has few cues to tell what an image is about without <code>@alt</code>, and other users 258may have no way of understanding what video or audio contents are about either.</p> 259 260<p>(For images whose <code>alt</code> attributes would introduce redundancy, and for images 261whose purpose is purely decorative which you cannot immediately use CSS for, use 262no alternative text, as in <code>alt=""</code>.)</p> 263 264<p></p> 265 266<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 267<img src="spreadsheet.png"> 268</code></pre> 269 270<pre><code class="language-html prettyprint"><!-- Recommended --> 271<img src="spreadsheet.png" alt="Spreadsheet screenshot."> 272</code></pre> 273 274<h4 id="Separation_of_Concerns">3.1.5 Separation of Concerns</h4> 275 276<p>Separate structure from presentation from behavior.</p> 277 278<p>Strictly keep structure (markup), presentation (styling), and behavior 279(scripting) apart, and try to keep the interaction between the three to an 280absolute minimum.</p> 281 282<p>That is, make sure documents and templates contain only HTML and HTML that is 283solely serving structural purposes. Move everything presentational into style 284sheets, and everything behavioral into scripts.</p> 285 286<p>In addition, keep the contact area as small as possible by linking as few style 287sheets and scripts as possible from documents and templates.</p> 288 289<p>Separating structure from presentation from behavior is important for 290maintenance reasons. It is always more expensive to change HTML documents and 291templates than it is to update style sheets and scripts.</p> 292 293<p></p> 294 295<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 296<!DOCTYPE html> 297<title>HTML sucks</title> 298<link rel="stylesheet" href="base.css" media="screen"> 299<link rel="stylesheet" href="grid.css" media="screen"> 300<link rel="stylesheet" href="print.css" media="print"> 301<h1 style="font-size: 1em;">HTML sucks</h1> 302<p>I’ve read about this on a few sites but now I’m sure: 303 <u>HTML is stupid!!1</u> 304<center>I can’t believe there’s no way to control the styling of 305 my website without doing everything all over again!</center> 306</code></pre> 307 308<pre><code class="language-html prettyprint"><!-- Recommended --> 309<!DOCTYPE html> 310<title>My first CSS-only redesign</title> 311<link rel="stylesheet" href="default.css"> 312<h1>My first CSS-only redesign</h1> 313<p>I’ve read about this on a few sites but today I’m actually 314 doing it: separating concerns and avoiding anything in the HTML of 315 my website that is presentational. 316<p>It’s awesome! 317</code></pre> 318 319<h4 id="Entity_References">3.1.6 Entity References</h4> 320 321<p>Do not use entity references.</p> 322 323<p>There is no need to use entity references like <code>&mdash;</code>, <code>&rdquo;</code>, or 324<code>&#x263a;</code>, assuming the same encoding (UTF-8) is used for files and editors 325as well as among teams.</p> 326 327<p>The only exceptions apply to characters with special meaning in HTML (like <code><</code> 328and <code>&</code>) as well as control or “invisible” characters (like no-break spaces).</p> 329 330<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 331The currency symbol for the Euro is &ldquo;&eur;&rdquo;. 332</code></pre> 333 334<pre><code class="language-html prettyprint"><!-- Recommended --> 335The currency symbol for the Euro is “€”. 336</code></pre> 337 338<h4 id="Optional_Tags">3.1.7 Optional Tags</h4> 339 340<p>Omit optional tags (optional).</p> 341 342<p>For file size optimization and scannability purposes, consider omitting optional 343tags. The <a href="https://whatwg.org/specs/web-apps/current-work/multipage/syntax.html#syntax-tag-omission">HTML5 specification</a> 344defines what tags can be omitted.</p> 345 346<p>(This approach may require a grace period to be established as a wider guideline 347as it’s significantly different from what web developers are typically taught. 348For consistency and simplicity reasons it’s best served omitting all optional 349tags, not just a selection.)</p> 350 351<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 352<!DOCTYPE html> 353<html> 354 <head> 355 <title>Spending money, spending bytes</title> 356 </head> 357 <body> 358 <p>Sic.</p> 359 </body> 360</html> 361</code></pre> 362 363<pre><code class="language-html prettyprint"><!-- Recommended --> 364<!DOCTYPE html> 365<title>Saving money, saving bytes</title> 366<p>Qed. 367</code></pre> 368 369<h4 id="type_Attributes">3.1.8 <code>type</code> Attributes</h4> 370 371<p>Omit <code>type</code> attributes for style sheets and scripts.</p> 372 373<p>Do not use <code>type</code> attributes for style sheets (unless not using CSS) and scripts 374(unless not using JavaScript).</p> 375 376<p>Specifying <code>type</code> attributes in these contexts is not necessary as HTML5 implies 377<a href="https://whatwg.org/specs/web-apps/current-work/multipage/semantics.html#attr-style-type"><code>text/css</code></a> 378and <a href="https://whatwg.org/specs/web-apps/current-work/multipage/scripting.html#attr-script-type"><code>text/javascript</code></a> 379as defaults. This can be safely done even for older browsers.</p> 380 381<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 382<link rel="stylesheet" href="https://www.google.com/css/maia.css" 383 type="text/css"> 384</code></pre> 385 386<pre><code class="language-html prettyprint"><!-- Recommended --> 387<link rel="stylesheet" href="https://www.google.com/css/maia.css"> 388</code></pre> 389 390<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 391<script src="https://www.google.com/js/gweb/analytics/autotrack.js" 392 type="text/javascript"></script> 393</code></pre> 394 395<pre><code class="language-html prettyprint"><!-- Recommended --> 396<script src="https://www.google.com/js/gweb/analytics/autotrack.js"></script> 397</code></pre> 398 399<h3 id="HTML_Formatting_Rules">3.2 HTML Formatting Rules</h3> 400 401<h4 id="General_Formatting">3.2.1 General Formatting</h4> 402 403<p>Use a new line for every block, list, or table element, and indent every such 404child element.</p> 405 406<p>Independent of the styling of an element (as CSS allows elements to assume a 407different role per <code>display</code> property), put every block, list, or table element 408on a new line.</p> 409 410<p>Also, indent them if they are child elements of a block, list, or table element.</p> 411 412<p>(If you run into issues around whitespace between list items it’s acceptable to 413put all <code>li</code> elements in one line. A linter is encouraged to throw a warning 414instead of an error.)</p> 415 416<pre><code class="language-html prettyprint"><blockquote> 417 <p><em>Space</em>, the final frontier.</p> 418</blockquote> 419</code></pre> 420 421<pre><code class="language-html prettyprint"><ul> 422 <li>Moe 423 <li>Larry 424 <li>Curly 425</ul> 426</code></pre> 427 428<pre><code class="language-html prettyprint"><table> 429 <thead> 430 <tr> 431 <th scope="col">Income 432 <th scope="col">Taxes 433 <tbody> 434 <tr> 435 <td>$ 5.00 436 <td>$ 4.50 437</table> 438</code></pre> 439 440<h4 id="HTML_Quotation_Marks">3.2.2 HTML Quotation Marks</h4> 441 442<p>When quoting attributes values, use double quotation marks.</p> 443 444<p>Use double (<code>""</code>) rather than single quotation marks (<code>''</code>) around attribute 445values.</p> 446 447<pre><code class="language-html prettyprint badcode"><!-- Not recommended --> 448<a class='maia-button maia-button-secondary'>Sign in</a> 449</code></pre> 450 451<pre><code class="language-html prettyprint"><!-- Recommended --> 452<a class="maia-button maia-button-secondary">Sign in</a> 453</code></pre> 454 455<h2 id="CSS">4 CSS</h2> 456 457<h3 id="CSS_Style_Rules">4.1 CSS Style Rules</h3> 458 459<h4 id="CSS_Validity">4.1.1 CSS Validity</h4> 460 461<p>Use valid CSS where possible.</p> 462 463<p>Unless dealing with CSS validator bugs or requiring proprietary syntax, use 464valid CSS code.</p> 465 466<p> 467 468Use tools such as the <a href="https://jigsaw.w3.org/css-validator/">W3C CSS validator</a> 469to test. 470</p> 471 472<p>Using valid CSS is a measurable baseline quality attribute that allows to spot 473CSS code that may not have any effect and can be removed, and that ensures 474proper CSS usage.</p> 475 476<h4 id="ID_and_Class_Naming">4.1.2 ID and Class Naming</h4> 477 478<p>Use meaningful or generic ID and class names.</p> 479 480<p>Instead of presentational or cryptic names, always use ID and class names that 481reflect the purpose of the element in question, or that are otherwise generic.</p> 482 483<p>Names that are specific and reflect the purpose of the element should be 484preferred as these are most understandable and the least likely to change.</p> 485 486<p>Generic names are simply a fallback for elements that have no particular or no 487meaning different from their siblings. They are typically needed as “helpers.”</p> 488 489<p>Using functional or generic names reduces the probability of unnecessary 490document or template changes.</p> 491 492<pre><code class="language-css prettyprint badcode">/* Not recommended: meaningless */ 493#yee-1901 {} 494 495/* Not recommended: presentational */ 496.button-green {} 497.clear {} 498</code></pre> 499 500<pre><code class="language-css prettyprint">/* Recommended: specific */ 501#gallery {} 502#login {} 503.video {} 504 505/* Recommended: generic */ 506.aux {} 507.alt {} 508</code></pre> 509 510<h4 id="ID_and_Class_Name_Style">4.1.3 ID and Class Name Style</h4> 511 512<p>Use ID and class names that are as short as possible but as long as necessary.</p> 513 514<p>Try to convey what an ID or class is about while being as brief as possible.</p> 515 516<p>Using ID and class names this way contributes to acceptable levels of 517understandability and code efficiency.</p> 518 519<pre><code class="language-css prettyprint badcode">/* Not recommended */ 520#navigation {} 521.atr {} 522</code></pre> 523 524<pre><code class="language-css prettyprint">/* Recommended */ 525#nav {} 526.author {} 527</code></pre> 528 529<h4 id="Type_Selectors">4.1.4 Type Selectors</h4> 530 531<p>Avoid qualifying ID and class names with type selectors.</p> 532 533<p>Unless necessary (for example with helper classes), do not use element names in 534conjunction with IDs or classes.</p> 535 536<p>Avoiding unnecessary ancestor selectors is useful for <a href="http://www.stevesouders.com/blog/2009/06/18/simplifying-css-selectors/">performance reasons</a>.</p> 537 538<pre><code class="language-css prettyprint badcode">/* Not recommended */ 539ul#example {} 540div.error {} 541</code></pre> 542 543<pre><code class="language-css prettyprint">/* Recommended */ 544#example {} 545.error {} 546</code></pre> 547 548<h4 id="Shorthand_Properties">4.1.5 Shorthand Properties</h4> 549 550<p>Use shorthand properties where possible.</p> 551 552<p>CSS offers a variety of <a href="https://www.w3.org/TR/CSS21/about.html#shorthand">shorthand</a> 553properties (like <code>font</code>) that should be used whenever possible, even in cases 554where only one value is explicitly set.</p> 555 556<p>Using shorthand properties is useful for code efficiency and understandability.</p> 557 558<pre><code class="language-css prettyprint badcode">/* Not recommended */ 559border-top-style: none; 560font-family: palatino, georgia, serif; 561font-size: 100%; 562line-height: 1.6; 563padding-bottom: 2em; 564padding-left: 1em; 565padding-right: 1em; 566padding-top: 0; 567</code></pre> 568 569<pre><code class="language-css prettyprint">/* Recommended */ 570border-top: 0; 571font: 100%/1.6 palatino, georgia, serif; 572padding: 0 1em 2em; 573</code></pre> 574 575<h4 id="0_and_Units">4.1.6 0 and Units</h4> 576 577<p>Omit unit specification after “0” values, unless required.</p> 578 579<p>Do not use units after <code>0</code> values unless they are required.</p> 580 581<pre><code class="language-css prettyprint">flex: 0px; /* This flex-basis component requires a unit. */ 582flex: 1 1 0px; /* Not ambiguous without the unit, but needed in IE11. */ 583margin: 0; 584padding: 0; 585</code></pre> 586 587<h4 id="Leading_0s">4.1.7 Leading 0s</h4> 588 589<p>Omit leading “0”s in values.</p> 590 591<p>Do not use put <code>0</code>s in front of values or lengths between -1 and 1.</p> 592 593<pre><code class="language-css prettyprint">font-size: .8em; 594</code></pre> 595 596<h4 id="Hexadecimal_Notation">4.1.8 Hexadecimal Notation</h4> 597 598<p>Use 3 character hexadecimal notation where possible.</p> 599 600<p>For color values that permit it, 3 character hexadecimal notation is shorter and 601more succinct.</p> 602 603<pre><code class="language-css prettyprint badcode">/* Not recommended */ 604color: #eebbcc; 605</code></pre> 606 607<pre><code class="language-css prettyprint">/* Recommended */ 608color: #ebc; 609</code></pre> 610 611<h4 id="Prefixes">4.1.9 Prefixes</h4> 612 613<p>Prefix selectors with an application-specific prefix (optional).</p> 614 615<p>In large projects as well as for code that gets embedded in other projects or on 616external sites use prefixes (as namespaces) for ID and class names. Use short, 617unique identifiers followed by a dash.</p> 618 619<p></p> 620 621<p></p> 622 623<p>Using namespaces helps preventing naming conflicts and can make maintenance 624easier, for example in search and replace operations.</p> 625 626<pre><code class="language-css prettyprint">.adw-help {} /* AdWords */ 627#maia-note {} /* Maia */ 628</code></pre> 629 630<h4 id="ID_and_Class_Name_Delimiters">4.1.10 ID and Class Name Delimiters</h4> 631 632<p>Separate words in ID and class names by a hyphen.</p> 633 634<p>Do not concatenate words and abbreviations in selectors by any characters 635(including none at all) other than hyphens, in order to improve understanding 636and scannability.</p> 637 638<pre><code class="language-css prettyprint badcode">/* Not recommended: does not separate the words “demo” and “image” */ 639.demoimage {} 640 641/* Not recommended: uses underscore instead of hyphen */ 642.error_status {} 643</code></pre> 644 645<pre><code class="language-css prettyprint">/* Recommended */ 646#video-id {} 647.ads-sample {} 648</code></pre> 649 650<h4 id="Hacks">4.1.11 Hacks</h4> 651 652<p>Avoid user agent detection as well as CSS “hacks”—try a different approach 653first.</p> 654 655<p>It’s tempting to address styling differences over user agent detection or 656special CSS filters, workarounds, and hacks. Both approaches should be 657considered last resort in order to achieve and maintain an efficient and 658manageable code base. Put another way, giving detection and hacks a free pass 659will hurt projects in the long run as projects tend to take the way of least 660resistance. That is, allowing and making it easy to use detection and hacks 661means using detection and hacks more frequently—and more frequently is too 662frequently.</p> 663 664<p></p> 665 666<p></p> 667 668<h3 id="CSS_Formatting_Rules">4.2 CSS Formatting Rules</h3> 669 670<h4 id="Declaration_Order">4.2.1 Declaration Order</h4> 671 672<p>Alphabetize declarations.</p> 673 674<p>Put declarations in alphabetical order in order to achieve consistent code in a 675way that is easy to remember and maintain.</p> 676 677<p>Ignore vendor-specific prefixes for sorting purposes. However, multiple 678vendor-specific prefixes for a certain CSS property should be kept sorted (e.g. 679-moz prefix comes before -webkit).</p> 680 681<pre><code class="language-css prettyprint">background: fuchsia; 682border: 1px solid; 683-moz-border-radius: 4px; 684-webkit-border-radius: 4px; 685border-radius: 4px; 686color: black; 687text-align: center; 688text-indent: 2em; 689</code></pre> 690 691<h4 id="Block_Content_Indentation">4.2.2 Block Content Indentation</h4> 692 693<p>Indent all block content.</p> 694 695<p>Indent all <a href="https://www.w3.org/TR/CSS21/syndata.html#block">block content</a>, 696that is rules within rules as well as declarations, so to reflect hierarchy and 697improve understanding.</p> 698 699<pre><code class="language-css prettyprint">@media screen, projection { 700 701 html { 702 background: #fff; 703 color: #444; 704 } 705 706} 707</code></pre> 708 709<h4 id="Declaration_Stops">4.2.3 Declaration Stops</h4> 710 711<p>Use a semicolon after every declaration.</p> 712 713<p>End every declaration with a semicolon for consistency and extensibility 714reasons.</p> 715 716<pre><code class="language-css prettyprint badcode">/* Not recommended */ 717.test { 718 display: block; 719 height: 100px 720} 721</code></pre> 722 723<pre><code class="language-css prettyprint">/* Recommended */ 724.test { 725 display: block; 726 height: 100px; 727} 728</code></pre> 729 730<h4 id="Property_Name_Stops">4.2.4 Property Name Stops</h4> 731 732<p>Use a space after a property name’s colon.</p> 733 734<p>Always use a single space between property and value (but no space between 735property and colon) for consistency reasons.</p> 736 737<pre><code class="language-css prettyprint badcode">/* Not recommended */ 738h3 { 739 font-weight:bold; 740} 741</code></pre> 742 743<pre><code class="language-css prettyprint">/* Recommended */ 744h3 { 745 font-weight: bold; 746} 747</code></pre> 748 749<h4 id="Declaration_Block_Separation">4.2.5 Declaration Block Separation</h4> 750 751<p>Use a space between the last selector and the declaration block.</p> 752 753<p>Always use a single space between the last selector and the opening brace that 754begins the <a href="https://www.w3.org/TR/CSS21/syndata.html#rule-sets">declaration block</a>.</p> 755 756<p>The opening brace should be on the same line as the last selector in a given 757rule.</p> 758 759<pre><code class="language-css prettyprint badcode">/* Not recommended: missing space */ 760#video{ 761 margin-top: 1em; 762} 763 764/* Not recommended: unnecessary line break */ 765#video 766{ 767 margin-top: 1em; 768} 769</code></pre> 770 771<pre><code class="language-css prettyprint">/* Recommended */ 772#video { 773 margin-top: 1em; 774} 775</code></pre> 776 777<h4 id="Selector_and_Declaration_Separation">4.2.6 Selector and Declaration Separation</h4> 778 779<p>Separate selectors and declarations by new lines.</p> 780 781<p>Always start a new line for each selector and declaration.</p> 782 783<pre><code class="language-css prettyprint badcode">/* Not recommended */ 784a:focus, a:active { 785 position: relative; top: 1px; 786} 787</code></pre> 788 789<pre><code class="language-css prettyprint">/* Recommended */ 790h1, 791h2, 792h3 { 793 font-weight: normal; 794 line-height: 1.2; 795} 796</code></pre> 797 798<h4 id="Rule_Separation">4.2.7 Rule Separation</h4> 799 800<p>Separate rules by new lines.</p> 801 802<p>Always put a blank line (two line breaks) between rules.</p> 803 804<pre><code class="language-css prettyprint">html { 805 background: #fff; 806} 807 808body { 809 margin: auto; 810 width: 50%; 811} 812</code></pre> 813 814<h4 id="CSS_Quotation_Marks">4.2.8 CSS Quotation Marks</h4> 815 816<p>Use single quotation marks for attribute selectors and property values.</p> 817 818<p>Use single (<code>''</code>) rather than double (<code>""</code>) quotation marks for attribute 819selectors or property values. Do not use quotation marks in URI values 820(<code>url()</code>).</p> 821 822<p>Exception: If you do need to use the <code>@charset</code> rule, use double quotation 823marks—<a href="https://www.w3.org/TR/CSS21/syndata.html#charset">single quotation marks are not permitted</a>.</p> 824 825<pre><code class="language-css prettyprint badcode">/* Not recommended */ 826@import url("https://www.google.com/css/maia.css"); 827 828html { 829 font-family: "open sans", arial, sans-serif; 830} 831</code></pre> 832 833<pre><code class="language-css prettyprint">/* Recommended */ 834@import url(https://www.google.com/css/maia.css); 835 836html { 837 font-family: 'open sans', arial, sans-serif; 838} 839</code></pre> 840 841<h3 id="CSS_Meta_Rules">4.3 CSS Meta Rules</h3> 842 843<h4 id="Section_Comments">4.3.1 Section Comments</h4> 844 845<p>Group sections by a section comment (optional).</p> 846 847<p>If possible, group style sheet sections together by using comments. Separate 848sections with new lines.</p> 849 850<pre><code class="language-css prettyprint">/* Header */ 851 852#adw-header {} 853 854/* Footer */ 855 856#adw-footer {} 857 858/* Gallery */ 859 860.adw-gallery {} 861</code></pre> 862 863 864 865<p></p> 866 867<p></p> 868 869 870 871<p></p> 872 873<p></p> 874 875<pre></pre> 876 877 878 879 880 881<p></p> 882 883<p></p> 884 885<h2 id="Parting_Words">Parting Words</h2> 886 887<p><em>Be consistent.</em></p> 888 889<p>If you’re editing code, take a few minutes to look at the code around you and 890determine its style. If they use spaces around all their arithmetic operators, 891you should too. If their comments have little boxes of hash marks around them, 892make your comments have little boxes of hash marks around them too.</p> 893 894<p>The point of having style guidelines is to have a common vocabulary of coding so 895people can concentrate on what you’re saying rather than on how you’re saying 896it. We present global style rules here so people know the vocabulary, but local 897style is also important. If code you add to a file looks drastically different 898from the existing code around it, it throws readers out of their rhythm when 899they go to read it. Avoid this.</p> 900</div> 901</body> 902</html> 903
