forked from google/styleguide
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathhtmlcssguide.html
959 lines (695 loc) · 31.4 KB
/
htmlcssguide.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Google HTML/CSS Style Guide</title>
<link rel="stylesheet" href="javaguide.css">
<script src="include/styleguide.js"></script>
<link rel="shortcut icon" href="https://www.google.com/favicon.ico">
<script src="include/jsguide.js"></script>
</head>
<body onload="initStyleGuide();">
<div id="content">
<h1>Google HTML/CSS Style Guide</h1>
<h2 id="Background" class="numbered">Background</h2>
<p>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.</p>
<h2 id="General" class="numbered">General</h2>
<h3 id="General_Style_Rules" class="numbered">General Style Rules</h3>
<h4 id="Protocol" class="numbered">Protocol</h4>
<p>Use HTTPS for embedded resources where possible.</p>
<p>Always use HTTPS (<code>https:</code>) for images and other media files, style sheets, and
scripts, unless the respective files are not available over HTTPS.</p>
<pre><code class="language-html bad"><!-- 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>
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.0/jquery.min.js"></script>
</code></pre>
<pre><code class="language-css bad">/* 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';
</code></pre>
<pre><code class="language-css good">/* Recommended */
@import 'https://fonts.googleapis.com/css?family=Open+Sans';
</code></pre>
<h3 id="General_Formatting_Rules" class="numbered">General Formatting Rules</h3>
<h4 id="Indentation" class="numbered">Indentation</h4>
<p>Indent by 2 spaces at a time.</p>
<p>Don’t use tabs or mix tabs and spaces for indentation.</p>
<pre><code class="language-html good"><ul>
<li>Fantastic
<li>Great
</ul>
</code></pre>
<pre><code class="language-css good">.example {
color: blue;
}
</code></pre>
<h4 id="Capitalization" class="numbered">Capitalization</h4>
<p>Use only lowercase.</p>
<p>All code has to be lowercase: This applies to HTML element names, attributes,
attribute values (unless <code>text/CDATA</code>), CSS selectors, properties, and property
values (with the exception of strings).</p>
<pre><code class="language-html bad"><!-- Not recommended -->
<A HREF="/">Home</A>
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
<img src="google.png" alt="Google">
</code></pre>
<pre><code class="language-css bad">/* Not recommended */
color: #E5E5E5;
</code></pre>
<pre><code class="language-css good">/* Recommended */
color: #e5e5e5;
</code></pre>
<h4 id="Trailing_Whitespace" class="numbered">Trailing Whitespace</h4>
<p>Remove trailing white spaces.</p>
<p>Trailing white spaces are unnecessary and can complicate diffs.</p>
<pre><code class="language-html bad"><!-- Not recommended -->
<p>What?_
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
<p>Yes please.
</code></pre>
<h3 id="General_Meta_Rules" class="numbered">General Meta Rules</h3>
<h4 id="Encoding" class="numbered">Encoding</h4>
<p>Use UTF-8 (no BOM).</p>
<p>Make sure your editor uses UTF-8 as character encoding, without a byte order
mark.</p>
<p>Specify the encoding in HTML templates and documents via <code><meta
charset="utf-8"></code>. Do not specify the encoding of style sheets as these assume
UTF-8.</p>
<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 character encodings in HTML and CSS</a>.)</p>
<h4 id="Comments" class="numbered">Comments</h4>
<p>Explain code as needed, where possible.</p>
<p>Use comments to explain code: What does it cover, what purpose does it serve,
why is respective solution used or preferred?</p>
<p>(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.)</p>
<h4 id="Action_Items" class="numbered">Action Items</h4>
<p>Mark todos and action items with <code>TODO</code>.</p>
<p>Highlight todos by using the keyword <code>TODO</code> only, not other common formats like
<code>@@</code>.</p>
<p>Append a contact (username or mailing list) in parentheses as with the format
<code>TODO(contact)</code>.</p>
<p>Append action items after a colon as in <code>TODO: action item</code>.</p>
<pre><code class="language-django external good">{# TODO(john.doe): revisit centering #}
<center>Test</center>
</code></pre>
<pre><code class="language-html good"><!-- TODO: remove optional tags -->
<ul>
<li>Apples</li>
<li>Oranges</li>
</ul>
</code></pre>
<h2 id="HTML" class="numbered">HTML</h2>
<h3 id="HTML_Style_Rules" class="numbered">HTML Style Rules</h3>
<h4 id="Document_Type" class="numbered">Document Type</h4>
<p>Use HTML5.</p>
<p>HTML5 (HTML syntax) is preferred for all HTML documents: <code><!DOCTYPE html></code>.</p>
<p>(It’s recommended to use HTML, as <code>text/html</code>. Do not use XHTML. XHTML, as
<a href="https://hixie.ch/advocacy/xhtml"><code>application/xhtml+xml</code></a>, lacks both browser
and infrastructure support and offers less room for optimization than HTML.)</p>
<p>Although fine with HTML, do not close void elements, i.e. write <code><br></code>, not <code><br
/></code>.</p>
<h4 id="HTML_Validity" class="numbered">HTML Validity</h4>
<p>Use valid HTML where possible.</p>
<p>Use valid HTML code unless that is not possible due to otherwise unattainable
performance goals regarding file size.</p>
<p> Use tools such as the
<a href="https://validator.w3.org/nu/">W3C HTML validator</a>
to test.</p>
<p>Using valid HTML is a measurable baseline quality attribute that contributes to
learning about technical requirements and constraints, and that ensures proper
HTML usage.</p>
<pre><code class="language-html bad"><!-- Not recommended -->
<title>Test</title>
<article>This is only a test.
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
<!DOCTYPE html>
<meta charset="utf-8">
<title>Test</title>
<article>This is only a test.</article>
</code></pre>
<h4 id="Semantics" class="numbered">Semantics</h4>
<p>Use HTML according to its purpose.</p>
<p>Use elements (sometimes incorrectly called “tags”) for what they have been
created for. For example, use heading elements for headings, <code>p</code> elements for
paragraphs, <code>a</code> elements for anchors, etc.</p>
<p>Using HTML according to its purpose is important for accessibility, reuse, and
code efficiency reasons.</p>
<pre><code class="language-html bad"><!-- Not recommended -->
<div onclick="goToRecommendations();">All recommendations</div>
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
<a href="recommendations/">All recommendations</a>
</code></pre>
<h4 id="Multimedia_Fallback" class="numbered">Multimedia Fallback</h4>
<p>Provide alternative contents for multimedia.</p>
<p>For multimedia, such as images, videos, animated objects via <code>canvas</code>, make sure
to offer alternative access. For images that means use of meaningful alternative
text (<code>alt</code>) and for video and audio transcripts and captions, if available.</p>
<p>Providing alternative contents is important for accessibility reasons: A blind
user has few cues to tell what an image is about without <code>@alt</code>, and other users
may have no way of understanding what video or audio contents are about either.</p>
<p>(For images whose <code>alt</code> 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 <code>alt=""</code>.)</p>
<pre><code class="language-html bad"><!-- Not recommended -->
<img src="spreadsheet.png">
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
<img src="spreadsheet.png" alt="Spreadsheet screenshot.">
</code></pre>
<h4 id="Separation_of_Concerns" class="numbered">Separation of Concerns</h4>
<p>Separate structure from presentation from behavior.</p>
<p>Strictly keep structure (markup), presentation (styling), and behavior
(scripting) apart, and try to keep the interaction between the three to an
absolute minimum.</p>
<p>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.</p>
<p>In addition, keep the contact area as small as possible by linking as few style
sheets and scripts as possible from documents and templates.</p>
<p>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.</p>
<pre><code class="language-html bad"><!-- 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>
</code></pre>
<pre><code class="language-html good"><!-- 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!
</code></pre>
<h4 id="Entity_References" class="numbered">Entity References</h4>
<p>Do not use entity references.</p>
<p>There is no need to use entity references like <code>&mdash;</code>, <code>&rdquo;</code>, or
<code>&#x263a;</code>, assuming the same encoding (UTF-8) is used for files and editors as
well as among teams.</p>
<p>The only exceptions apply to characters with special meaning in HTML (like <code><</code>
and <code>&</code>) as well as control or “invisible” characters (like no-break spaces).</p>
<pre><code class="language-html bad"><!-- Not recommended -->
The currency symbol for the Euro is &ldquo;&eur;&rdquo;.
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
The currency symbol for the Euro is “€”.
</code></pre>
<h4 id="Optional_Tags" class="numbered">Optional Tags</h4>
<p>Omit optional tags (optional).</p>
<p>For file size optimization and scannability purposes, consider omitting optional
tags. The
<a href="https://html.spec.whatwg.org/multipage/syntax.html#syntax-tag-omission">HTML5 specification</a>
defines what tags can be omitted.</p>
<p>(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.)</p>
<pre><code class="language-html bad"><!-- Not recommended -->
<!DOCTYPE html>
<html>
<head>
<title>Spending money, spending bytes</title>
</head>
<body>
<p>Sic.</p>
</body>
</html>
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
<!DOCTYPE html>
<title>Saving money, saving bytes</title>
<p>Qed.
</code></pre>
<h4 id="type_Attributes" class="numbered"><code>type</code> Attributes</h4>
<p>Omit <code>type</code> attributes for style sheets and scripts.</p>
<p>Do not use <code>type</code> attributes for style sheets (unless not using CSS) and scripts
(unless not using JavaScript).</p>
<p>Specifying <code>type</code> attributes in these contexts is not necessary as HTML5 implies
<a href="https://html.spec.whatwg.org/multipage/obsolete.html#attr-style-type"><code>text/css</code></a>
and
<a href="https://html.spec.whatwg.org/multipage/scripting.html#attr-script-type"><code>text/javascript</code></a>
as defaults. This can be safely done even for older browsers.</p>
<pre><code class="language-html bad"><!-- Not recommended -->
<link rel="stylesheet" href="https://www.google.com/css/maia.css"
type="text/css">
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
<link rel="stylesheet" href="https://www.google.com/css/maia.css">
</code></pre>
<pre><code class="language-html bad"><!-- Not recommended -->
<script src="https://www.google.com/js/gweb/analytics/autotrack.js"
type="text/javascript"></script>
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
<script src="https://www.google.com/js/gweb/analytics/autotrack.js"></script>
</code></pre>
<h4 id="id_Attributes" class="numbered"><code>id</code> Attributes</h4>
<p>Avoid unnecessary <code>id</code> attributes.</p>
<p>Prefer <code>class</code> attributes for styling and <code>data</code> attributes for scripting.</p>
<p>Where <code>id</code> attributes are strictly required, always include a hyphen in the
value to ensure it does not match the JavaScript identifier syntax, e.g. use
<code>user-profile</code> rather than just <code>profile</code> or <code>userProfile</code>.</p>
<p>When an element has an <code>id</code> attribute, browsers will make that available as a
<a href="https://html.spec.whatwg.org/multipage/window-object.html#named-access-on-the-window-object">named property on the global <code>window</code> prototype</a>,
which may cause unexpected behavior. While <code>id</code> attribute values containing a
hyphen are still available as property names, these cannot be referenced as
global JavaScript variables.</p>
<pre><code class="language-html bad"><!-- Not recommended: `window.userProfile` will resolve to reference the <div> node -->
<div id="userProfile"></div>
</code></pre>
<pre><code class="language-html good"><!-- Recommended: `id` attribute is required and its value includes a hyphen -->
<div aria-describedby="user-profile">
…
<div id="user-profile"></div>
…
</div>
</code></pre>
<h3 id="HTML_Formatting_Rules" class="numbered">HTML Formatting Rules</h3>
<h4 id="General_Formatting" class="numbered">General Formatting</h4>
<p>Use a new line for every block, list, or table element, and indent every such
child element.</p>
<p>Independent of the styling of an element (as CSS allows elements to assume a
different role per <code>display</code> property), put every block, list, or table element
on a new line.</p>
<p>Also, indent them if they are child elements of a block, list, or table element.</p>
<p>(If you run into issues around whitespace between list items it’s acceptable to
put all <code>li</code> elements in one line. A linter is encouraged to throw a warning
instead of an error.)</p>
<pre><code class="language-html good"><blockquote>
<p><em>Space</em>, the final frontier.</p>
</blockquote>
</code></pre>
<pre><code class="language-html good"><ul>
<li>Moe
<li>Larry
<li>Curly
</ul>
</code></pre>
<pre><code class="language-html good"><table>
<thead>
<tr>
<th scope="col">Income
<th scope="col">Taxes
<tbody>
<tr>
<td>$ 5.00
<td>$ 4.50
</table>
</code></pre>
<h4 id="HTML_Line-Wrapping" class="numbered">HTML Line-Wrapping</h4>
<p>Break long lines (optional).</p>
<p>While there is no column limit recommendation for HTML, you may consider
wrapping long lines if it significantly improves readability.</p>
<p>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.</p>
<pre><code class="language-html good"><button
mat-icon-button
color="primary"
class="menu-button"
(click)="openMenu()"
>
<mat-icon>menu</mat-icon>
</button>
</code></pre>
<pre><code class="language-html good"><button mat-icon-button color="primary" class="menu-button"
(click)="openMenu()">
<mat-icon>menu</mat-icon>
</button>
</code></pre>
<pre><code class="language-html good"><button
mat-icon-button
color="primary"
class="menu-button"
(click)="openMenu()">
<mat-icon>menu</mat-icon>
</button>
</code></pre>
<pre><code class="language-html good"><button mat-icon-button
color="primary"
class="menu-button"
(click)="openMenu()">
<mat-icon>menu</mat-icon>
</button>
</code></pre>
<h4 id="HTML_Quotation_Marks" class="numbered">HTML Quotation Marks</h4>
<p>When quoting attributes values, use double quotation marks.</p>
<p>Use double (<code>""</code>) rather than single quotation marks (<code>''</code>) around attribute
values.</p>
<pre><code class="language-html bad"><!-- Not recommended -->
<a class='maia-button maia-button-secondary'>Sign in</a>
</code></pre>
<pre><code class="language-html good"><!-- Recommended -->
<a class="maia-button maia-button-secondary">Sign in</a>
</code></pre>
<h2 id="CSS" class="numbered">CSS</h2>
<h3 id="CSS_Style_Rules" class="numbered">CSS Style Rules</h3>
<h4 id="CSS_Validity" class="numbered">CSS Validity</h4>
<p>Use valid CSS where possible.</p>
<p>Unless dealing with CSS validator bugs or requiring proprietary syntax, use
valid CSS code.</p>
<p> Use tools such as the
<a href="https://jigsaw.w3.org/css-validator/">W3C CSS validator</a>
to test.</p>
<p>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.</p>
<h4 id="ID_and_Class_Naming" class="numbered">Class Naming</h4>
<p>Use meaningful or generic class names.</p>
<p>Instead of presentational or cryptic names, always use class names that reflect
the purpose of the element in question, or that are otherwise generic.</p>
<p>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.</p>
<p>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.”</p>
<p>Using functional or generic names reduces the probability of unnecessary
document or template changes.</p>
<pre><code class="language-css bad">/* Not recommended: meaningless */
.yee-1901 {}
/* Not recommended: presentational */
.button-green {}
.clear {}
</code></pre>
<pre><code class="language-css good">/* Recommended: specific */
.gallery {}
.login {}
.video {}
/* Recommended: generic */
.aux {}
.alt {}
</code></pre>
<h4 id="ID_and_Class_Name_Style" class="numbered">Class Name Style</h4>
<p>Use class names that are as short as possible but as long as necessary.</p>
<p>Try to convey what a class is about while being as brief as possible.</p>
<p>Using class names this way contributes to acceptable levels of understandability
and code efficiency.</p>
<pre><code class="language-css bad">/* Not recommended */
.navigation {}
.atr {}
</code></pre>
<pre><code class="language-css good">/* Recommended */
.nav {}
.author {}
</code></pre>
<h4 id="ID_and_Class_Name_Delimiters" class="numbered">Class Name Delimiters</h4>
<p>Separate words in class names by a hyphen.</p>
<p>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.</p>
<pre><code class="language-css bad">/* Not recommended: does not separate the words “demo” and “image” */
.demoimage {}
/* Not recommended: uses underscore instead of hyphen */
.error_status {}
</code></pre>
<pre><code class="language-css good">/* Recommended */
.video-id {}
.ads-sample {}
</code></pre>
<h4 id="Prefixes" class="numbered">Prefixes</h4>
<p>Prefix selectors with an application-specific prefix (optional).</p>
<p>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.</p>
<p>Using namespaces helps preventing naming conflicts and can make maintenance
easier, for example in search and replace operations.</p>
<pre><code class="language-css good">.adw-help {} /* AdWords */
.maia-note {} /* Maia */
</code></pre>
<h4 id="Type_Selectors" class="numbered">Type Selectors</h4>
<p>Avoid qualifying class names with type selectors.</p>
<p>Unless necessary (for example with helper classes), do not use element names in
conjunction with classes.</p>
<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>
<pre><code class="language-css bad">/* Not recommended */
ul.example {}
div.error {}
</code></pre>
<pre><code class="language-css good">/* Recommended */
.example {}
.error {}
</code></pre>
<h4 id="ID_Selectors" class="numbered">ID Selectors</h4>
<p>Avoid ID selectors.</p>
<p>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.</p>
<pre><code class="language-css bad">/* Not recommended */
#example {}
</code></pre>
<pre><code class="language-css good">/* Recommended */
.example {}
</code></pre>
<h4 id="Shorthand_Properties" class="numbered">Shorthand Properties</h4>
<p>Use shorthand properties where possible.</p>
<p>CSS offers a variety of
<a href="https://www.w3.org/TR/CSS21/about.html#shorthand">shorthand</a>
properties (like <code>font</code>) that should be used whenever possible, even in cases
where only one value is explicitly set.</p>
<p>Using shorthand properties is useful for code efficiency and understandability.</p>
<pre><code class="language-css bad">/* 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;
</code></pre>
<pre><code class="language-css good">/* Recommended */
border-top: 0;
font: 100%/1.6 palatino, georgia, serif;
padding: 0 1em 2em;
</code></pre>
<h4 id="0_and_Units" class="numbered">0 and Units</h4>
<p>Omit unit specification after “0” values, unless required.</p>
<p>Do not use units after <code>0</code> values unless they are required.</p>
<pre><code class="language-css good">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;
</code></pre>
<h4 id="Leading_0s" class="numbered">Leading 0s</h4>
<p>Always include leading “0”s in values.</p>
<p>Put <code>0</code>s in front of values or lengths between -1 and 1.</p>
<pre><code class="language-css good">font-size: 0.8em;
</code></pre>
<h4 id="Hexadecimal_Notation" class="numbered">Hexadecimal Notation</h4>
<p>Use 3 character hexadecimal notation where possible.</p>
<p>For color values that permit it, 3 character hexadecimal notation is shorter and
more succinct.</p>
<pre><code class="language-css bad">/* Not recommended */
color: #eebbcc;
</code></pre>
<pre><code class="language-css good">/* Recommended */
color: #ebc;
</code></pre>
<h4 id="Important_Declarations" class="numbered">Important Declarations</h4>
<p>Avoid using <code>!important</code> declarations.</p>
<p>These declarations break the natural cascade of CSS and make it difficult to
reason about and compose styles. Use
<a href="https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity">selector specificity</a>
to override properties instead.</p>
<pre><code class="language-css bad">/* Not recommended */
.example {
font-weight: bold !important;
}
</code></pre>
<pre><code class="language-css good">/* Recommended */
.example {
font-weight: bold;
}
</code></pre>
<h4 id="Hacks" class="numbered">Hacks</h4>
<p>Avoid user agent detection as well as CSS “hacks”—try a different approach
first.</p>
<p>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.</p>
<h3 id="CSS_Formatting_Rules" class="numbered">CSS Formatting Rules</h3>
<h4 id="Declaration_Order" class="numbered">Declaration Order</h4>
<p>Alphabetize declarations (optional).</p>
<p>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.</p>
<p>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).</p>
<pre><code class="language-css good">background: fuchsia;
border: 1px solid;
-moz-border-radius: 4px;
-webkit-border-radius: 4px;
border-radius: 4px;
color: black;
text-align: center;
text-indent: 2em;
</code></pre>
<h4 id="Block_Content_Indentation" class="numbered">Block Content Indentation</h4>
<p>Indent all block content.</p>
<p>Indent all
<a href="https://www.w3.org/TR/CSS21/syndata.html#block">block content</a>,
that is rules within rules as well as declarations, so to reflect hierarchy and
improve understanding.</p>
<pre><code class="language-css good">@media screen, projection {
html {
background: #fff;
color: #444;
}
}
</code></pre>
<h4 id="Declaration_Stops" class="numbered">Declaration Stops</h4>
<p>Use a semicolon after every declaration.</p>
<p>End every declaration with a semicolon for consistency and extensibility
reasons.</p>
<pre><code class="language-css bad">/* Not recommended */
.test {
display: block;
height: 100px
}
</code></pre>
<pre><code class="language-css good">/* Recommended */
.test {
display: block;
height: 100px;
}
</code></pre>
<h4 id="Property_Name_Stops" class="numbered">Property Name Stops</h4>
<p>Use a space after a property name’s colon.</p>
<p>Always use a single space between property and value (but no space between
property and colon) for consistency reasons.</p>
<pre><code class="language-css bad">/* Not recommended */
h3 {
font-weight:bold;
}
</code></pre>
<pre><code class="language-css good">/* Recommended */
h3 {
font-weight: bold;
}
</code></pre>
<h4 id="Declaration_Block_Separation" class="numbered">Declaration Block Separation</h4>
<p>Use a space between the last selector and the declaration block.</p>
<p>Always use a single space between the last selector and the opening brace that
begins the
<a href="https://www.w3.org/TR/CSS21/syndata.html#rule-sets">declaration block</a>.</p>
<p>The opening brace should be on the same line as the last selector in a given
rule.</p>
<pre><code class="language-css bad">/* Not recommended: missing space */
.video{
margin-top: 1em;
}
/* Not recommended: unnecessary line break */
.video
{
margin-top: 1em;
}
</code></pre>
<pre><code class="language-css good">/* Recommended */
.video {
margin-top: 1em;
}
</code></pre>
<h4 id="Selector_and_Declaration_Separation" class="numbered">Selector and Declaration Separation</h4>
<p>Separate selectors and declarations by new lines.</p>
<p>Always start a new line for each selector and declaration.</p>
<pre><code class="language-css bad">/* Not recommended */
a:focus, a:active {
position: relative; top: 1px;
}
</code></pre>
<pre><code class="language-css good">/* Recommended */
h1,
h2,
h3 {
font-weight: normal;
line-height: 1.2;
}
</code></pre>
<h4 id="Rule_Separation" class="numbered">Rule Separation</h4>
<p>Separate rules by new lines.</p>
<p>Always put a blank line (two line breaks) between rules.</p>
<pre><code class="language-css good">html {
background: #fff;
}
body {
margin: auto;
width: 50%;
}
</code></pre>
<h4 id="CSS_Quotation_Marks" class="numbered">CSS Quotation Marks</h4>
<p>Use single (<code>''</code>) rather than double (<code>""</code>) quotation marks for attribute
selectors and property values.</p>
<p>Do not use quotation marks in URI values (<code>url()</code>).</p>
<p>Exception: If you do need to use the <code>@charset</code> rule, use double quotation
marks—<a href="https://www.w3.org/TR/CSS21/syndata.html#charset">single quotation marks are not permitted</a>.</p>
<pre><code class="language-css bad">/* Not recommended */
@import url("https://www.google.com/css/maia.css");
html {
font-family: "open sans", arial, sans-serif;
}
</code></pre>
<pre><code class="language-css good">/* Recommended */
@import url(https://www.google.com/css/maia.css);
html {
font-family: 'open sans', arial, sans-serif;
}
</code></pre>
<h3 id="CSS_Meta_Rules" class="numbered">CSS Meta Rules</h3>
<h4 id="Section_Comments" class="numbered">Section Comments</h4>
<p>Group sections by a section comment (optional).</p>
<p>If possible, group style sheet sections together by using comments. Separate
sections with new lines.</p>
<pre><code class="language-css good">/* Header */
.adw-header {}
/* Footer */
.adw-footer {}
/* Gallery */
.adw-gallery {}
</code></pre>
<h2 id="Parting_Words">Parting Words</h2>
<p><em>Be consistent.</em></p>
<p>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.</p>
<p>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.</p>
</div>
</body>
</html>