-
Notifications
You must be signed in to change notification settings - Fork 790
Expand file tree
/
Copy pathOverview.bs
More file actions
519 lines (436 loc) · 16.8 KB
/
Overview.bs
File metadata and controls
519 lines (436 loc) · 16.8 KB
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
<style>
.example .figure img {
background-color: white;
padding: 20px;
margin: 20px;
}
</style>
<pre class=metadata>
Title: CSS Rhythmic Sizing
Shortname: css-rhythm
Level: 1
Group: CSSWG
Status: ED
Work Status: exploring
Editor: Koji Ishii, Google, kojiishi@gmail.com, w3cid 45369
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
ED: https://drafts.csswg.org/css-rhythm/
TR: https://www.w3.org/TR/css-rhythm-1/
Abstract: This module contains CSS features for aligning content size
to multiple of unit size.
</pre>
<pre class=link-defaults>
spec:css2; type:property; text:max-height
spec:css2; type:property; text:max-width
spec:css2; type:property; text:min-width
spec:css-writing-modes-4; type:dfn; text:end
spec:css-writing-modes-4; type:dfn; text:start
spec:css-break-3; type:dfn; text:fragment
</pre>
Introduction {#intro}
=====================
This specification provides features to control sizes of CSS objects
according to the rules desired by use cases.
Controlling sizes of CSS objects to be multiple of a unit
is desired in many cases.
This level of the specification focuses on following cases.
* Adjust heights of block-level boxes to multiples of the specified length.
* Adjust heights of line boxes to multiples of the specified length.
By stacking blocks at multiples,
authors can align content across columns, pages, scroll-snapped bocks
or multiple blocks placed absolutely
to maintain vertical rhythm.
Also by controlling heights of line boxes,
lines of text in different fonts can create consistent visuals
to help readability.
<div class="figure">
<a href="examples/snap-height.html">
<img src="images/snap-height-sample.png"></a>
<p class="caption">Vertical rhythm kept through pictures and different size of text in a multi-column document.
</div>
East Asian Casual Vertical Rhythms {#eastasia}
----------------------------------------------
In East Asia, a casual variant of vertical rhythm is widely used.
Vertical rhythm is typically used in professional typography.
While it improves readability,
its spacing constraints require careful and well-thought design of spaces.
The East Asian casual variant was
originally a product of technical constraints of traditional word processors in '80s.
But when the technical constraints were lifted in more modern technologies,
with the help of square-like visual of ideographic characters,
East Asian authors preferred to keep parts of the characteristics.
In this variant of vertical rhythm,
the requirement is loosened for the ease of use for non-professional authors.
Text is still on the rhythm,
so that the majority of ideographic characters are mostly on grid,
but when author specifies borders, margins, or some other objects that may break the rhythm,
the rhythm is shifted rather than forced.
The strict vertical rhythm often surprises non-professional authors by forced jumps in such cases,
while this variant combines rhythm on text and the ease of use for non-professional authors.
This variant was very widely accepted in East Asia since the middle of '90s,
such that most major word processors used in East Asia provided similar features
by default.
In East Asian publishing typography,
the vertical rhythm is one of important properties,
but its priority compared to other properties
varies by types of documents.
In single column documents,
the priority is weaker than that of multi-column documents.
Text should be on the vertical rhythm,
but it is often preferred for borders, margins,
or other properties to win over the rhythm.
In such cases, the rhythm is shifted, similar to the casual variant.
In this specification,
when the 'line-height-step' property is used without combination of the 'block-step' property
or the 'line-grid' property,
it produces the similar effect as the East Asian casual vertical rhythm.
It may also serve good for East Asian publishing typography,
depends on the desired strength of the vertical rhythm.
For other cases of vertical rhythm,
it is expected that
the 'block-step' property or the 'line-grid' property are used,
or that the 'line-height-step' property is used together with them.
Adjusting Block-level Box Heights {#block-height}
=================================
The most common typographic rhythm problems are block-level intrusions:
when continuous paragraph-level text is interrupted
by differently-sized content such as illustrations and headings,
then the line-to-line rhythm can be thrown off.
The block step properties allow such elements
to be fitted to the rhythmic interval
by constraining their height to a multiple of a specified step size.
This allows content before and after the interruption
to maintain a continuous rhythm.
<div class="figure">
<img src="images/block-step-size-before.png"
style="width: 500px">
<figcaption>Heads and blockquotes have varying font sizes and line heights,
resulting in uneven text across columns.</figcaption>
</div>
<div class="figure">
<img src="images/block-step-size-after.png"
style="width: 500px">
<figcaption>Space inserted before and after blocks (shown with colored borders)
restores vertical rhythm.</figcaption>
</div>
While consistent use of the block step properties
can produce the strictly gridded layout needed
for parallel content flows,
per-element specification of the step size
also allows some interruptions to take their natural size in the flow,
restarting the vertical rhythm afterwards.
This can be useful in single-column layouts
where the vertical rhythm is important to maintain visual continuity,
but there is nothing alongside to align to.
In these casese,
large interruptions which visually disconnect the flow before and after
can prioritize optimal spacing around the item
over strict adherence to a continouous grid
(by specifying ''block-step: none'', the initial value).
ISSUE: This proposal can be simplified down to just the 'block-step-size' property, represented solely through its shortened form as 'block-step'.
This level will likely at most contain 'block-step-size' and 'block-step-insert', leaving 'block-step-align' and 'block-step-round' to be added if the future demands.
The full design is described herein for current discussion and future reference.
ISSUE: This proposal is currently defined to apply only to block-level boxes. This limitation is solely to simplify the first iteration; it should eventually be extended to all layout modes that honor specified heights.
Specifying the Step Size: the 'block-step-size' property {#block-step-size}
--------------------------------------------------------
<pre class="propdef">
Name: block-step-size
Value: none | <<length>>
Initial: none
Applies to: block-level boxes
Inherited: no
Animatable: ???
Percentages: N/A
Media: visual
Computed Value: keyword or absolute length
</pre>
This property defines the <a>step unit</a> for a block-level box’s <a>block size</a>.
When the <a>step unit</a> is set to a positive <<length>>,
the box’s outer height is rounded
(see 'block-step-round')
to the closest multiple of the unit.
Negative <<length>> values are invalid.
Values other than ''block-step-size/none''
cause the box to [=establish an independent formatting context=].
In situations where margins <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapse</a>,
only the box’s own margin is considered
in calculating its outer size.
When a box <a>fragments</a>,
step sizing is applied per fragment.
Specifying the Spacing Type: the 'block-step-insert' property {#block-step-insert}
--------------------------------------------------------
<pre class="propdef">
Name: block-step-insert
Value: margin | padding
Initial: margin
Applies to: block-level boxes
Inherited: no
Animatable: no
Percentages: N/A
Media: visual
Computed Value: as specified
</pre>
This property specifies whether extra spacing
derived from applying 'block-step-size'
is inserted inside (like 'padding') or outside (like 'margin')
the box’s border.
Values have the following meanings:
<dl dfn-for="block-step-insert" dfn-type="value">
<dt><dfn>margin</dfn>
<dd>
Any extra space resulting from a 'block-step-size'-induced adjustment
is inserted outside the box’s border, as extra margin.
<dt><dfn>padding</dfn>
<dd>
Any extra space resulting from a 'block-step-size'-induced adjustment
is inserted inside the box’s border, as extra padding.
</dl>
Specifying Alignment: the 'block-step-align' property {#block-step-align}
-----------------------------------------------------
<pre class="propdef">
Name: block-step-align
Value: auto | center | start | end
Initial: auto
Applies to: block-level boxes
Inherited: no
Animatable: no
Percentages: N/A
Media: visual
Computed Value: as specified
</pre>
This property specifies whether extra spacing
derived from applying 'block-step-size'
is inserted before, inserted after, or split between both sides of the box.
Values have the following meanings:
<dl dfn-for="block-step-align" dfn-type="value">
<dt><dfn>auto</dfn>
<dd>
If 'block-step-insert' is ''margin'':
if 'align-self' is ''start'', ''end'', or ''center'', treat as that value,
otherwise treat as ''center''.
<dt><dfn>center</dfn>
<dd>
Any extra space resulting from a 'block-step-size'-induced adjustment
is split, and applied half on either side of the box.
<dt><dfn>start</dfn>
<dd>
Any extra space resulting from a 'block-step-size'-induced adjustment
is inserted on the <a>end</a> side of the box.
<dt><dfn>end</dfn>
<dd>
Any extra space resulting from a 'block-step-size'-induced adjustment
is inserted on the <a>start</a> side of the box.
</dl>
In all cases,
additional spacing cannot be added to margins
that are truncated or omitted due to unforced fragmentation breaks
(see [[CSS3-PAGE]] and [[CSS-BREAK-3]]);
therefore if 'block-step-insert' is ''margin'',
all extra space derived from applying 'block-step-size'
must be inserted on the opposite side of the fragment
(regardless of 'block-step-align').
Rounding Method: the 'block-step-round' property {#block-step-round}
------------------------------------------------
<pre class="propdef">
Name: block-step-round
Value: up | down | nearest
Initial: up
Applies to: block-level boxes
Inherited: no
Animatable: no
Percentages: N/A
Media: visual
Computed Value: as specified
</pre>
This property specifies whether adjustments due to 'block-step-size'
insert positive or negative space.
Values have the following meanings:
<dl dfn-for="block-step-align" dfn-type="value">
<dt><dfn>up</dfn>
<dd>
The outer size of the box is <em>increased</em>
(positive space is inserted)
to fulfill the 'block-step-size' constraint.
<dt><dfn>down</dfn>
<dd>
The outer size of the box is <em>decreased</em>
(negative space is inserted)
to fulfill the 'block-step-size' constraint.
<dt><dfn>nearest</dfn>
<dd>
The outer size of the box is either
<em>increased</em> (as for ''up'') or <em>decreased</em> (as for ''down''--
whichever results in the smallest absolute change--
to fulfill the 'block-step-size' constraint.
If both options would result in the same amount of change,
the size is increased.
</dl>
Block Step Adjustment Shorthand: the 'block-step' shorthand {#block-step}
-----------------------------------------------------------
<pre class="propdef">
Name: block-step
Value: <'block-step-size'> || <'block-step-insert'> || <'block-step-align'> || <'block-step-round'>
Initial: See individual properties
Applies to: block-level boxes
Inherited: no
Animatable: See individual properties
Percentages: N/A
Media: visual
Computed Value: See individual properties
</pre>
This <a>shorthand property</a> allows for setting
'block-step-size',
'block-step-insert',
'block-step-align',
and
'block-step-round'
in one declaration.
Omitted values are set to the property’s initial value.
Advisement: Authors are advised to use this shorthand rather than the longhands
unless there is a specific need for its individual longhands to cascade independently.
Adjusting Line Box Heights: the 'line-height-step' property {#line-height-step}
===============================================================================
<pre class='propdef'>
Name: line-height-step
Value: <<length>>
Initial: 0
Applies to: block containers
Inherited: yes
Animatable: no
Percentages: N/A
Media: visual
Computed Value: the absolute length
</pre>
This property defines the <dfn>step unit</dfn> for line box heights.
A <a>step unit</a> is the length of the vertical rhythm,
usually the distance from one baseline to the next baseline of the body text.
Body text should fit into one <a>step unit</a>,
and taller lines such as headings
should have heights of two or more <a>step unit</a>s.
Vertical rhythm is created
by making heights of all lines an integer multiple of the <a>step unit</a>.
When the <a>step unit</a> is set to a positive <<length>>,
the line box heights are rounded <i>up</i> to
the closest multiple of the unit.
Negative <<length>> values are invalid.
[[!CSS21]] <a href="https://drafts.csswg.org/css2/visudet.html#line-height">§10.8 Line height calculations</a>
defines how to compute the height of a line box from its inline-level content.
The rounding is applied to the resulting height of the line box,
and the additional space is distributed to
<a>over</a>-side and <a>under</a>-side of the line box equally,
so that the original line box appears at the center of the
multiple of <a>step unit</a>.
This adjustment is done
by assuming that there is an inline-level box that has adjusted A' and D'
in the line box.
This inline-level box does not affect alignment points of the 'vertical-align' property,
except values that align relative to the line box.
<div class="figure">
<img src="images/adjust-line-height.svg">
<p class="caption">Rounding up the computed line box height.
</div>
<div class="issue">Should this be animatable?
There doesn't seem to be use cases but needed for consistency?</div>
<div class="example">
<div class="figure" style="float:right">
<img src="images/line-grid-center.svg"
style="height: 300px"
>
</div>
In the following example,
the height of line box in each paragraph is rounded up to the <a>step unit</a>.
<pre class="lang-css">
:root {
font-size: 12pt;
--my-grid: 18pt;
line-height-step: var(--my-grid);
}
h1 {
font-size: 20pt;
margin-top: calc(2 * var(--my-grid));
}
p {
margin: 0;
}
</pre>
The line box in <code><h1></code> does not fit into one <a>step unit</a>
and thus occupies two,
but it is still centered within the two <a>step unit</a>.
</div>
<div class="example">
Authors can keep margins or other properties to be multiple of <a>step unit</a>
using ''var()'' and ''calc()'' as in the example above.
If author prefers,
tools like Sass can make such declarations shorter.
<pre class="lang-css">
$gu: 18px;
@function gu($n) {
@return $n * $gu;
}
h1 {
font-size: 20pt;
margin: gu(1.2) auto gu(1.8);
}
</pre>
</div>
<div class="note">
It is usually recommended to set the 'line-height' lower than
the <a>step unit</a>.
The used line height can increase due to several factors such as
the use of 'vertical-align' or font fallback.
</div>
<!--
Notes on Block-level Boxes {#inline-block}
------------------------------------------
<i>This section is not normative.</i>
This level of the specification does not provide features
to adjust heights of block-level boxes.
<div class="example">
The following CSS turns <code><h2></code> to inline-blocks,
so that the 'line-height-step' property can control its height.
<pre class="lang-css">
:root {
line-height-step: 18pt;
}
h2 {
display: inline-block;
width: 100%;
line-height-step: 0;
line-height: 1.2;
}
</pre>
When an <code><h2></code> is long enough to wrap,
text inside the <code><h2></code> uses ''line-height: 1.2'', while
the height of the <code><h2></code> block is rounded up
to the multiple of ''18pt''.
See <a href="examples/snap-height.html">a sample in action</a>.
</div>
-->
Privacy and Security Considerations {#priv-sec}
===============================================
This specification introduces no new privacy leaks,
or security considerations beyond "implement it correctly".
Acknowledgments {#acks}
=======================
This specification would not have been possible without the help from:
Masaharu Akutsu,
Yoko Aoki,
Takao Baba,
Chris Eppstein,
Ichiro Inaba,
Jxck,
Noriko Kase,
Motoya Kinoshita,
Shinyu Murakami,
Tsutomu Nanjo,
Kiyoshi Narishima,
Charlie Neely,
Takuya Nishimura,
Katsuhiro Osumi,
Florian Rivoal,
Hiroshi Sakakibara,
Alan Stearns,
Masataka Yakura,
KADOKAWA Corporation,
PixelGrid Inc.,
and the CSS Working Group members.