CSS Inline Layout Module Level 3

Editor’s Draft,

Specification Metadata
This version:
https://drafts.csswg.org/css-inline-3/
Latest published version:
https://www.w3.org/TR/css-inline-3/
Previous Versions:
Issue Tracking:
Inline In Spec
GitHub Issues
Editors:
(Hachette Livre)
Elika J. Etemad / fantasai (Invited Expert)
(Adobe)
Suggest an Edit for this Spec:
GitHub Editor
Issues list:
CSS3 Line Layout issues in GitHub

Abstract

The CSS formatting model provides for a flow of elements and text inside of a container to be wrapped into lines. The formatting of elements and text within a line, its positioning in the inline progression direction, and the breaking of lines are described in [CSS-TEXT-3]. This module describes the positioning in the block progression direction both of elements and text within lines and of the lines themselves. This positioning is often relative to a baseline. It also describes special features for formatting of first lines and drop caps. It extends on the model in [CSS2].

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-inline” in the title, preferably like this: “[css-inline] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 February 2018 W3C Process Document.

The following features are at-risk, and may be dropped during the CR period:

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

1. Introduction

This module defines the CSS Inline Layout model, replacing and extending the model as defined in CSS2.1. It is very much a work-in-progress, and implementers should reference CSS2.1 for now.

The root inline box is an anonymous inline box which is automatically generated to hold all of the inline-level contents of a block container (if it has any). It inherits from its parent block container, but is otherwise unstyleable.

Note: Line boxes, like column boxes [css-multicol-1], are fragmentation containers generated by their formatting context and are not part of the CSS box tree.

2. Line Heights and Baseline Alignment

This section is being rewritten. Refer to section 10.8 of [CSS2] for the normative CSS definition or the 2002 Working Draft if you want pretty pictures. (But ignore the old text, half of it’s wrong. We’re not specifying which half, that’s to be determined.) The CSS2 specification should be used as the guideline for implementation.

The CSSWG would like to know which baseline values are necessary: if any can be dropped, or any need to be added. See GitHub issue 859.

2.1. Dominant Baselines: the dominant-baseline property

Name: dominant-baseline
Value: auto | text-bottom | alphabetic | ideographic | middle | central | mathematical | hanging | text-top
Initial: normal
Applies to: block containers, inline boxes, table rows, table columns, grid containers, and flex containers
Inherited: yes
Percentages: N/A
Computed value: specified keyword
Canonical order: per grammar
Animation type: discrete

This property specifies the dominant baseline, which is the baseline used to align the box’s text and inline-level contents. It is also indicates the default alignment baseline of any boxes participating in baseline alignment in the box’s alignment context. Values have the following meanings:

auto
Equivalent to alphabetic in horizontal writing modes and in vertical writing modes when text-orientation is sideways, sideways-right, or sideways-left. Equivalent to central in vertical writing modes when text-orientation is mixed or upright.

However, in SVG text, the origin point of glyphs (used for coordinate-based glyph positioning) is always handled as for central in vertical writing modes.

text-bottom
Use the bottom of the em box as the baseline.
alphabetic
Use the alphabetic baseline.
ideographic
Match the box’s ideographic under-side baseline to that of its parent. This corresponds to the ideo baseline in OpenType.
middle
Use the “middle” baseline: halfway between the alphabetic baseline and the ex-height.
central
Use the central baseline (halfway between the ascent and descent).
mathematical
Use the mathematical baseline.
hanging
Use the hanging baseline.
text-top
Use the top of the em box as the baseline.

See [CSS-WRITING-MODES-3] for an introduction to dominant baselines.

Should be text-over and text-under instead of text-top and text-bottom, but maybe it’s better not to use those terms for consistency with legacy vertical-align. See GitHub issue 860.

Add first and last values. Note, in this property, these are combinatorial, whereas in the align/justify-self/content properties, it’s singular. Do we want to align the syntaxes wrt hyphens vs. spaces or what? See GitHub issue 861.

2.2. Transverse Box Alignment: the vertical-align property

Name: vertical-align
Value: <‘baseline-shift’> || <‘alignment-baseline’>
Initial: baseline
Applies to: inline-level boxes
Inherited: no
Percentages: N/A
Computed value: see individual properties
Animation type: see individual properties
Canonical order: per grammar

This shorthand property specifies how an inline-level box is aligned within the line. Values are the same as for its longhand properties, see below.

Authors should use this property (vertical-align) instead of its longhands.

This property will gain first and last keywords, like in the box alignment properties, see CSS Box Alignment 3 §4.2 Baseline Alignment: the baseline keyword and first/last modifiers. The open question is whether they should be added to alignment-baseline or a new sub-property should be created to hold the first | last preference.

2.2.1. Alignment Point: alignment-baseline longhand

Name: alignment-baseline
Value: baseline | text-bottom | alphabetic | ideographic | middle | central | mathematical | text-top | bottom | center | top
Initial: baseline
Applies to: inline-level boxes, flex items, grid items, table cells
Inherited: no
Percentages: N/A
Computed value: specified keyword
Canonical order: per grammar
Animation type: discrete

Specifies what point of an inline-level box is aligned to what point in the parent. Also selects the alignment baseline of boxes aligned with align-self/justify-self.

Clean up this prose to correctly handle alignment contexts other than inline formatting contexts.

Values are defined below:

For the following definitions, the margin box is used for atomic inlines, the leading box for non-replaced inlines, and the baselines of the box are synthesized if missing in the line-box’s inline axis:

baseline
Use the dominant baseline choice of the parent. Match the box’s corresponding baseline to that of its parent.
text-bottom
Match the bottom of the box to the bottom of the parent’s content area.
alphabetic
Match the box’s alphabetic baseline to that of its parent.
ideographic
Match the box’s ideographic character face under-side baseline to that of its parent.
middle
Align the vertical midpoint of the box with the baseline of the parent box plus half the x-height of the parent.
central
Match the box’s central baseline to the central baseline of its parent.
mathematical
Match the box’s mathematical baseline to that of its parent.
text-top
Match the top of the box to the top of the parent’s content area.

For the following definitions, the alignment subtree is as defined in [CSS2].

top
Align the top of the aligned subtree with the top of the line box.
center
Align the center of the aligned subtree with the center of the line box.
bottom
Align the bottom of the aligned subtree with the bottom of the line box.

SVG implementations may support the following aliases in order to support legacy content:

text-before-edge = text-top
text-after-edge = text-bottom

These values are not allowed in the vertical-align shorthand.

2.2.2. Alignment Shift: the baseline-shift longhand

Name: baseline-shift
Value: <length-percentage> | sub | super
Initial: 0
Applies to: inline-level boxes, flex items, grid items
Inherited: no
Percentages: refer to the used value of line-height
Computed value: the specified keyword and/or a computed <length-percentage> value
Canonical order: per grammar
Animation type: discrete

This property specifies by how much the box is shifted up from its alignment point. It does not apply when alignment-baseline is top or bottom.

Authors should use the vertical-align shorthand instead of this property.

Values have the following meanings:

<length>
Raise (positive value) or lower (negative value) by the specified length.
<percentage>
Raise (positive value) or lower (negative value) by the specified percentage of the line-height.
sub
Lower by the offset appropriate for subscripts of the parent’s box. (The UA should use the parent’s font data to find this offset whenever possible.)
super
Raise by the offset appropriate for superscripts of the parent’s box. (The UA should use the parent’s font data to find this offset whenever possible.)

User agents may additionally support the keyword baseline as computing to 0 if is necessary for them to support legacy SVG content.

We would prefer to remove this, and are looking for feedback from SVG user agents as to whether it’s necessary.

2.3. Line Spacing: the line-height property

Name: line-height
Value: normal | <number> || <length-percentage>
Initial: normal
Applies to: inline boxes
Inherited: yes
Percentages: computed relative to 1em
Computed value: the specified keyword, a number, or a computed <length> value
Canonical order: per grammar
Animation type: discrete
normal
<number>
<length-percentage>
See CSS2§10.8.1.

Fill out this definition so that it fully replaces CSS2.

The fact that percentages compute to lengths is annoying. See also href="https://github.com/w3c/csswg-drafts/issues/3118">Issue 3118 and Issue 2165.

2.4. Line Sizing Containment: the line-sizing property

Name: line-sizing
Value: quirks-behavior | current-behavior | better-behavior | box-model-behavior | absolute-behavior
Initial: current-behavior
Applies to: block containers? inline boxes?
Inherited: yes
Percentages: N/A
Computed value: the specified keyword
Canonical order: per grammar
Animation type: discrete

This is a rought draft of a proposal, and has not yet been approved by the CSSWG. Many variants have been included to promote discussion of possible behaviors; not all of them are expected to be kept. See discussion in Issue 3199, Hyatt’s message, and dbaron’s proposal. Also all the keywords are expected to be renamed.

This property controls the method by which line boxes are sized, and thus the spacing between lines of text.

Values have the following meanings:

quirks-behavior
Line boxes are sized, and content positioned within them, as defined in [CSS2] except that any inline box fragment that has zero borders and padding and that does not directly contain text or preserved white space [CSS-TEXT-3] is ignored for this purpose.
current-behavior
Line boxes are sized, and content positioned within them, as defined in [CSS2].

Note: In this model, vertical rhythm is broken any time there is a change in font metrics within a paragraph.

better-behavior
Line boxes are sized, and content positioned within them, as defined in [CSS2], except that positive half-leading is not applied to any box other than the root inline box.

Note: This will give consistent line spacing as long as there is some amount of leading added, such that the half-leading on the root inline is large enough to accommodate the unleaded ascent of its descendants. The line box will grow to accommodate content that would otherwise overflow, avoiding overlap between lines.

box-model-behavior
Line boxes are sized to fit the root inline box and its half-leading, as well as the outer edges of any inline-level descendants in the same inline formatting context. Positive half-leading is not applied to any other inline box; negative half-leading is applied as negative margins to inline boxes whose block-axis margins, borders, and padding are zero.

Note: This mode is similar to better-behavior, but re-uses the familiar margin/border/padding box model of controlling spacing. However to ensure that line-height < 1 behaves as expected, the default case (no margin/border/padding) needs to apply negative leading to such inlines.

absolute-behavior
Line boxes are sized to fit the root inline box and its half-leading; no other boxes are considered. Inline-level content may overflow the line box and overlap adjacent lines if it extends higher or lower than the edges of the root inline box’s leading.

Should this property apply to block containers or to inline boxes? In the latter case, an individual inline could say "pay attention to me" or "don’t pay attention to me".

Need better names. Maybe `loose | normal | strict | absolute`? Maybe `quirks | legacy | normal | absolute`? Something else?

3. Drawing Inline Boxes

3.1. Inline Box Heights: the inline-sizing property

Name: inline-sizing
Value: normal | stretch
Initial: normal
Applies to: inline boxes
Inherited: no
Percentages: n/a
Computed value: specified keyword
Canonical order: per grammar
Animation type: discrete

This property specifies how the logical height of the content area of an inline box is measured and how it is aligned with its contents. (It has no effect on the size or position of the box’s contents.) Values have the following meanings:

normal
The content area of the inline box is sized and position to fit its (possibly hypothetical) text as specified in CSS2§10.6.1.
stretch
The logical height of the content area is calculated as the stretch fit into the line box. The box is then positioned such that its margin edges coincide with the line box’s edges.

We might want to use this opportunity to more precisely define normal, rename it to match, and possibly introduce any other values that may seem necessary.

4. Initial Letters

The editors would appreciate any examples of drop initials in non-western scripts, especially Arabic and Indic scripts.

4.1. An Introduction to Initial Letters

This section is non-normative.

Large, decorative letters have been used to start new sections of text since before the invention of printing. In fact, their use predates lowercase letters entirely.

4.1.1. Drop Initial

A dropped initial (or “drop cap”) is a larger-than-usual letter at the start of a paragraph, with a baseline at least one line lower than the first baseline of the paragraph. The size of the drop initial is usually indicated by how many lines it occupies. Two- and three-line drop initials are very common.

3-line drop cap with E Acute
Three-line drop initial with E acute. Since the cap-height of the drop initial aligns with the cap-height of the main text, the accent extends above the paragraph.

The exact size and position of a dropped initial depends on the alignment of its glyph. Reference points on the drop cap must align precisely with reference points in the text. The alignment constraints for drop initials depend on the writing system.

In Western scripts, the top reference points are the cap height of the initial letter and of the first line of text. The bottom reference points are the alphabetic baseline of the initial letter and the baseline of the Nth line of text. Figure 2 shows a simple two-line drop cap, with the relevant reference lines marked.

drop cap showing alignment
Two-line drop cap showing baselines (green lines), cap-height (red line), and ascender (cyan line).

In Han-derived scripts, the initial letter extends from the block-start edge of the glyphs on the first line to the block-end edge of the glyphs on the Nth line.

Japanese Vertical Initial
Two-line drop initial in vertical writing mode

In certain Indic scripts, the top alignment point is the hanging baseline, and the bottom alignment point is the text-after-edge.

Devangari initial letter
Devangari initial letter aligned with hanging baseline. Alignment points shown in red.

4.1.2. Sunken Initial Letters

Some styles of drop initials do not align with the first line of text. A sunken initial (or “sunken cap”) both sinks below the first baseline, and extends above the first line of text.

sunken drop initial
Sunken cap. The letter drops two lines, but is the size of a three-line initial letter.

4.1.3. Raised Initial Letters

A raised initial (often called a “raised cap” or “stick-up cap”) “sinks” to the first text baseline.

Note: A proper raised initial has several advantages over simply increasing the font size of a first letter. The line spacing in the rest of the paragraph will not be altered, but text will still be excluded around large descenders. And if the size of raised initial is defined to be an integral number of lines, implicit baseline grids can be maintained.

raised cap
Raised cap. The initial letter is the size of a 3-line initial, but does not drop.

4.2. Selecting Initial Letters

This section is non-normative.

Initial letters are typically a single letter, although they may include punctuation or a sequence of characters which are perceived by the user to be a single typographic unit. The ::first-letter pseudo-element, defined in [SELECT] and [CSS-PSEUDO-4], can be used to select the character(s) to be formatted as initial letters.

Authors who need more control over which characters are included in an initial letter, or who want to apply initial-letters formatting to replaced elements or multiple words can alternately apply the initial-letters property to the first inline-level child of a block container.

<p>This paragraph has a dropped “T”.
<p><img alt="H" src="illuminated-h.svg">ere we have an illuminated “H”.
<p><span>Words may also</span> be given initial letter styling at the beginning of a paragraph.
::first-letter, /* style first paragraph’s T */
img, /* style illuminated H */
span /* style phrase inside span */
{ initial-letters: 2; }

Note that since ::first-letter selects punctuation before or after the first letter, these characters are included in the initial-letters when ::first-letter is used.

Paragraph showing both opening quote and first letter set as three-line drop cap
The ::first-letter pseudo-element selects the quotation mark as well as the “M”.

Should there be a way to opt out of this behavior? See Github Issue 310.

4.3. Creating Initial Letters: the initial-letters property

Name: initial-letters
Value: normal | <number> <integer> | <number> && [ drop | raise ]?
Initial: normal
Applies to: certain inline-level boxes and ::first-letter and inside ::marker boxes (see prose)
Inherited: no
Percentages: N/A
Computed value: the keyword normal or a number paired with an integer
Canonical order: per grammar
Animation type: by computed value type

Renaming this property (and the others in this section) is currently under discussion.

This property specifies the size and sink for dropped, raised, and sunken initial letters as the number of lines spanned.

For example, the following code will create a 2-line dropped initial letter at the beginning of each paragraph that immediately follows a second-level heading:
h2 + p::first-letter { initial-letters: 2; }

It takes the following values:

normal
No special initial-letters effect. Text behaves as normal.
<number>
This first argument defines the size of the initial letter in terms of how many lines it occupies. Values less than one are invalid.
<integer>
This optional second argument defines the number of lines the initial letter should sink. A value of 1 indicates a raised initial; values greater than 1 indiciate a sunken initial. Values less than one are invalid.
raise
Computes to an initial letter sink of 1.
drop
Computes to an initial letter sink equal to the initial letter size floored to the nearest positive whole number.

If the initial letter sink value is omitted, drop is assumed.

Values other than normal cause the affected box to become an initial letter box, which is an in-flow inline-level box with special layout behavior.

Here are some examples of initial-letters usage:
initial-letters: 3
initial-letters: 3 3
initial-letters: 3 drop
initial-letters: drop 3
Represents a dropped initial 3 lines high, 3 lines deep.

3 lines high, 3 lines deep

initial-letters: 3 2
Represents a sunken initial 3 lines high, 2 lines deep.

3 lines high, 2 lines deep

initial-letters: 3 1
initial-letters: 3 raise
initial-letters: raise 3
Represents a raised initial 3 lines high, 1 line deep.

3 lines high, 1 line deep

initial-letters: 2.51 3
The size of the initial letter does not have to be an integral number of lines. In this case only the bottom aligns.

Non-integral initial letter that only aligns at base

In conjunction with other CSS properties, initial-letters can be used to create “adjacent initial letters,” where the initial letter is adjacent to the text:
p::first-letter {
  initial-letters: 3;
  color: red;
  width: 5em;
  text-align: right;
  margin-left: -5em;
}

p {
  margin-left: 5em;
}
Initial letter adjacent to text

4.3.1. Applicability

To give authors more control over which characters can be styled as an initial letter and to allow the possibility of multi-character initial letters (such as for first word or first phrase styling), the initial-letters property applies not just to the CSS-defined ::first-letter pseudo-element, but also to inside-positioned ::marker pseudo-elements and inline-level boxes that are placed at the start of the first line. Specifically, initial-letters applies to any inline-level boxincluding any such ::first-letter or ::marker box—that is the first child of its parent box and whose ancestors (if any) that are descendants of its containing block are all first-child inline boxes that have a computed initial-letters value of normal.

For example, the <span>, <em>, and <b> elements in the following example are "first-most inline-level descendants" of the <p>, but the <strong> element is not:
<p><span><em><b>This</b> phrase</em> is styled
<strong>specially</strong>.</span> …

If we apply the following rules:

em { initial-letters: 2; }
b, strong { initial-letters: 3; }

The initial-letters property will take effect only on the <em>. The styling on <b> is ignored, as it has an ancestor already styled as an initial letter; and the styling on <strong> is ignored because it is a second sibling.

The result might be rendered as

“This phrase” becomes the dropped text spanning two lines, the remainder of the text wrapping alongside.

If initial-letters is applied to an inline-level box that is not positioned at the start of the line due to bidi reordering or which is otherwise preceded by other inline-level content, its used value is normal, and it is not formatted as an initial letter.

The effect of the initial-letters property is undefined on children of ruby base container boxes and on ruby container boxes.

Note: The initial-letters property cannot apply to any element whose float is not none or position is not static, because these values cause its display to compute to block.

4.4. Alignment of Initial Letters: the initial-letters-align property

As mentioned earlier, the alignment of initial letters depends on the script used. The initial-letters-align property can be used to specify the proper alignment.

Name: initial-letters-align
Value: border-box? [ alphabetic | ideographic | hebrew | hanging ] | border-box
Initial: alphabetic
Applies to: certain inline-level boxes and ::first-letter and inside ::marker boxes (see prose)
Inherited: yes
Percentages: N/A
Computed value: specified keyword(s)
Canonical order: per grammar
Animation type: discrete

This property specifies the alignment points used to size and position an initial letter. Two sets of alignment points are necessary: the over and under alignment points of the initial letter are matched to corresponding over and under points of the surrounding text.

Values have the following meanings:

alphabetic
Use the alphabetic and cap-height baselines of the surrounding text to align the initial letter.
ideographic
Use the ideographic character face bottom and top edge baselines of the surrounding text to align the initial letter.
hebrew
Use the alphabetic and (as yet theoretical) hebrew hanging baseline of the surrounding text to align the initial letter.
hanging
Use the alphabetic and hanging baselines of the surrounding text to align the initial letter.
border-box
Use the initial letter box’s line-over and line-under border edges as the over and under alignment points, respectively.

Is there a proper typographic term for the hebrew “hanging” baseline?

The vertical writing mode example from Figure 2 could be coded as:
span.initial {
  initial-letters: 2;
  initial-letters-align: ideographic;
}
Initial letter in Hebrew
span.initial {
initial-letters: 2;
initial-letters-align: hebrew;
}

optical kerning in the presence or absence of a space after the initial letter

Except when border-box is specified, the alignment points of the initial letter are automatically determined from its contents:

  1. If the initial letter is an atomic inline, use its over and under content-box edges.
  2. Else if the initial letter contains any character from the Han, Hangul, Kana, or Yi Unicode scripts, use the ideographic character face bottom and top edge baselines.
  3. Else if the initial letter contains any character from the Devanagari, Bengali, and Gurmukhi Unicode scripts, use the hanging and alphabetic baselines.
  4. Else if the initial letter contains any character from the Hebrew Unicode scripts, use the ideographic character face bottom and top edge baselines.
  5. Else use the alphabetic and cap-height baselines.

What is the proper alignment for South Asian scripts that do not have the explicit hanging baseline, such as Tamil or Telugu? See GitHub issue 864

Note: The ordering of keywords in this property is fixed in case border-box is expanded to [ border-box | alphabetic | ideographic | hebrew | hanging ] to allow explicitly specifying the initial letter’s alignment points.

4.4.1. UA Default Stylesheet for initial-letters-align

In order to provide the better behavior by default, UAs must include in their default UA style sheet the following rules:

[lang]:lang(zh, ja, ko, ii) {
  initial-letters-align: ideographic;
}
[lang]:lang(iw, yi, lad, jrb) {
  initial-letters-align: hebrew;
}
[lang]:lang(hi, mr, ne, pi, kok, brx, mai, sd, sa) {
  initial-letters-align: hanging;
}
/* Script tags override language tags */
[lang]:lang('*-Latn', '*-Cyrl') {
  initial-letters-align: alphabetic;
}
[lang]:lang('*-Hani', '*-Hant', '*-Hans') {
  initial-letters-align: ideographic;
}

This only covers the most common cross-linguistic transcription systems. Should we include any other / all script tags in the UA style sheet?

4.5. Initial Letter Layout

There are two types of initial letter boxes: those that arise from non-replaced inline boxes and those that arise from atomic inlines.

For the non-atomic inline initial letter, the box and its contents participate in the same inline formatting context as the line on which it occurs, and a lot of special rules apply to give the expected sizing and alignment.

For an atomic initial letter, however, which is either a replaced element or which establishes an independent formatting context for its contents, the sizing of the box (aside from its automatic size in the block axis) and layout of the contents within the box follows the usual rules: it is primarily the positioning of the box which is special.

4.5.1. Properties Applying to Initial Letters

All properties that apply to an inline box also apply to an inline initial letter except for vertical-align and its sub-properties, font-size, line-height, line-sizing, and inline-sizing. Additionally, all of the sizing properties and box-sizing also apply to initial letters, (see [css-sizing-3]).

All properties that apply to an atomic inline also apply to the atomic inline when styled as an initial letter, except for vertical-align and its sub-properties.

4.5.2. Margins, Borders, and Padding

Initial letters can be styled with margins, padding, and borders just like any other box. Unless initial-letters-align is border-box, its vertical alignment and sizing are not affected; however the effective exclusion area is (and corresponds to the margin area).

Reword that to handle box-sizing correctly.

When padding and borders are zero, the initial letter may be kerned; see below.

4.6. Font Sizing of Initial Letters

For an inline initial letter, the font size used for sizing the initial letter contents is calculated to fulfill its specified size (see initial-letters) as anchored by its specified alignment points (see initial-letters-align). Note that no layout is required in this calculation: it is based only on computed values and font metrics. These used font size calculations do not affect the computed font-size, and therefore have no effect on the computation of em length values, etc.

What about inheritance to descendants?

For an atomic initial letter, the used font size is the computed font size as usual.

The line height used in these calculations is the line-height of the containing block (or, in the case where a baseline grid is in use, the baseline-to-baseline spacing required by the baseline grid [CSS-LINE-GRID-1]). The contents of the lines spanned, and therefore any variation in their heights and positions, is not accounted for.

text underlay shows how initial letter alignment is not affected
     by the content of the spanned lines

For an N-line drop initial in a Western script, the cap-height of the letter needs to be (N – 1) times the line-height, plus the cap-height of the surrounding text. Note this height is not the font size of the drop initial.

Actually calculating this font size is tricky. For an N-line drop initial, we find the drop initial font size to be:

Font size of drop cap = ((N-1) * line-height + [cap-height of para] * [font size of paragraph])/[cap-height ratio of drop initial font]

Update this calculation to be a) generic across writing systems / alignment points and b) handle non-integer sizes.

A three-line drop initial in Adobe Minion Pro would have a font size of 61.2pt given 12pt text, 16pt line-height, and a cap-height of 651/1000 (from the font’s OS/2 table).

4.6.1. Shaping and Glyph Selection

When initial-letters is not normal, shaping should still occur across an inline initial letter box’s boundaries. (See CSS Text 3 §8.3 Shaping Across Element Boundaries.) For example, if the first letter of the Farsi word “پس” were styled with initial-letters: 2 1, both letters would be styled in their joined forms, with initial-form “ﭘ” as the initial letter, followed by the normally-styled final-form “ﺲ”. Note that the two letters might not always graphically connect, even when shaped in their joining forms.

Are there other things we need to consider here?

4.7. Sizing the Initial Letter Box

For an inline initial letter, if the initial letter’s specified width/specified height is definite, use that value (clamped as required by the min size and max size properties, and handling box-sizing as required) for that dimension of the box.

Otherwise it is considered to have an automatic size in that dimension and is sized and positioned to coincide with the smallest rectangle that would include the bounding boxes of all its glyphs—excluding any that hang (see hanging-punctuation)—as well as the margin boxes of any atomic inlines it contains.

Should the hanging punctuation be included in the box instead (so that the box is drawn around the punctuation when it is made visible through borders/background), but rather only excluded when positioning the box (so that the initial letter remains flush, with the hanging punctuation properly hanging)? See discussion.

For atomic initial letters, sizing follows the usual rules for that type of atomic inline. However, if the box has an automatic block size (auto), then its block size is determined as for an inline initial letter with border-box alignment, and is definite.

4.7.1. Alignment Within an Initial Letter Box

By default (i.e. under automatic sizing), the content box of an inline initial letter is fitted exactly to its content, and alignment properties like text-align or align-content do not apply. However, if the box is not sized automatically:

4.8. Initial Letter Positioning and Spacing

4.8.1. Space Below Initial Letters

The glyph(s) of an initial letter do not always fit within the specified sink. For example, if an initial letter has a descender, it could crash into the (n+1)th line of text. This is not desirable.

3-line drop cap with J, with descender crashing into fourth line of text
Incorrect: three-line initial letter with descender. In this font, the capital “J” extends well below the baseline (shown in red).

Therefore all line boxes impacted by the glyph bounding boxes of an initial letter are excluded, not just those within range of the initial letter sink. Specifically, for non-atomic initial letters, the content box of the element is sized to fit:

The initial letter is then made an exclusion area, the same as if it were a float. See initial-letter-wrap for details.

3-line drop cap with J, but four-line exclusion
Correct: text excluded around glyph bounding box

4.8.2. Block-axis Positioning

In the block axis, the initial letter is positioned as required to satisfy its under alignment point (initial-letters-align) at its specified sink (initial-letters), i.e. it is positioned such that it would sink the number of lines specified by initial-letters’s second argument and align to the requisite under alignment point if it was assumed that its containing block held only the initial letter itself followed by an infinite sequence of plain text as the direct contents of its root inline box.

Constant-sized text underlay shows how initial letter alignment
		          is not affected by the content of the spanned lines.

Its position is anchored with respect to the line on which it occurs.

4.8.3. Inline Positioning and Kerning

In the inline axis, the position of the inline letter is given by matching its inline-start margin edge with the inline-start edge of the line box.

However, if the initial letter is a non-atomic inline with an automatic inline size and zero padding and borders, it is negatively offset by the distance from the start edge of its content box to the point in the content that would have been placed at the start edge of the containing block if it were not an initial letter (i.e. the distance between its glyph bounding box and its start side bearing).

4.9. Initial Letter Wrapping: the initial-letters-wrap property

Note: initial-letters-wrap is at risk.

Name: initial-letters-wrap
Value: none | first | all | grid | <length-percentage>
Initial: none
Applies to: certain inline-level boxes and ::first-letter and inside ::marker boxes (see prose)
Inherited: yes
Percentages: relative to logical width of (last fragment of) initial letter
Computed value: specified keyword or computed <length-percentage> value
Canonical order: per grammar
Animation type: by computed value type

This property specifies whether lines impacted by an initial letter are shortened to fit the rectangular shape of the initial letter box or follow the contour of its end-edge glyph outline.

none
No contour-fitting is performed: each impacted line is aligned flush to the end margin edge of the initial letter.
first
Behaves as none if the first typographic character unit after the initial letter belongs to Unicode General Category Zs. Otherwise behaves as for all on the first line of the block containing the initial letter and as none on the rest.
This example shows why contour-fitting the first line is necessary, and why it is dropped when the initial letter is followed by a space:
optical kerning in the presence or absence of a space after the initial letter
In the top paragraph, the initial letter "A" has a word space after it: the gap between the top of the "A" and the next letter provides the necessary word separation. In the next paragraph, the initial letter "A" is part of the first word, and leaving a gap between the top of the "A" and the next letter would create a jarring visual break within the word. In this case, the first line of text should be kerned into the initial letter’s area, as shown in the bottom paragraph.

Do we need an unconditional first? (I.e. Should we rename this value to auto and add a first value that does not check for spaces?) See GitHub issue 410

all
For each line of text impacted by the initial letter, the line box adjacent to the initial letter starts at the start-most point that touches the ink of the initial letter, plus the amount of the initial letter’s end-side border+padding+margin.

If the value of shape-outside is not none, shape-outside is used instead of the glyph outline.

Note: This value is at-risk.

grid
This value is the same as none, except that the exclusion area of the impacted lines is increased as necessary for its end-edge to land on the character grid, i.e. to be a multiple of (1ic + letter-spacing) as computed on the containing block. The justify-self property can then be used to align the initial letter box within the exclusion area.
Diagram of Japanese initial letter in vertical writing mode
Diagram of Japanese initial letter in vertical writing mode

Note: In this example, the exclusion area for the drop initial is larger than its glyph in order to preserve inline-axis alignment.

Note: This value is also at-risk.

<length>
<percentage>
This value behaves the same as first except that the adjustment to the first line is given explicitly instead of being inferred from the glyph shape.

This really needs font-relative lengths to be relative to the used size.

Note: This value exists because it is easier to implement. Authors are encouraged to use the first value and to set margins to control spacing, and to use this as a fallback for glyph detection if necessary.

In the following example, UAs that support first will use the glyph outline plus the specified margin in order to place the first line, whereas UAs that only support <length> or <percentage> values will pull in the first line by 40% of the initial letter’s width (and then add the margin to that point).
h1 + p:first-letter {
  initial-letters: 3; /* 3-line drop-cap */
  initial-letters-wrap: first;
  margin-right: 0.1em;
}
@supports (not (initial-letters-wrap: first)) {
  /* Classes auto-generated on paragraphs to match first letter. */
  p.A:first-letter {
    initial-letters-wrap: -40%; /* Start of glyph outline, assuming correct font. */
  }
}

These values and related annoyance is likely unnecessary if someone submits a patch to Blink to support first.

Edit figure to show how auto behaves in varying contexts

p::first-letter {
  initial-letters: 3;
  initial-letters-wrap: none;
}
regular dropcap A
Ordinary initial letter with no wrapping.
p::first-letter {
  initial-letters: 3;
  initial-letters-wrap: all;
}

text wrapping around dropcap A

Text follows shape of initial letter. Each line box should just touch the ink of the letter, with some offset (represented by the shaded box).

p::first-letter {
  initial-letters: 3;
  initial-letters-wrap: first;
}

text wrapping around dropcap A but only on first line

Only the first line is moved up against the ink of the initial letter.

p::first-letter {
  initial-letters: 3;
  initial-letters-wrap: all;
}

text wrapping around dropcap V

text wrapping around dropcap P

text wrapping around dropcap W

4.10. Line Layout

An initial letter box is considered in-flow in its block formatting context, and is part of the contents of the line box in which it originates. Aside from vertical alignment, its interaction with the rest of the contents of the line is as normal, except in a few specific circumstances…

4.10.1. Inline Flow Layout: Alignment and Justification

For a raised initial no special consideration is given for alignment and justification: it is treated similar to any other inline-level content.

However, for a sunken initial its inline-start edge is anchored to the inline-start edge of the line box (after indentation) and text alignment affects the remaining content of the line in the remaining space, without moving the initial letter box itself.

Note: The CSSWG was not aware of any reasonable use cases for mixing non-start text alignment with dropped initial letters, and this was the most sensible behavior proposed. This behavior may be reconsidered if use cases require otherwise.

In addition, to ensure consistent alignment of all the impacted lines, any letter-spacing or justification opportunity that would normally be introduced by the juxtaposition of the contents of a sunken initial and the subsequent contents of the line is suppressed. (Note that this does not affect word-spacing or the justification opportunity introduced by a word separator because that space is provided by the typographic character unit alone and not by its juxtaposition with an adjacent character.)

4.10.2. Edge Effects: Indentation and Hanging Punctuation

text-indent and hanging-punctuation apply to the first line of text as normal in the presence of initial letters. Lines affected by the exclusion are shortened, as in the presence of floats, and are affected the same way.

initial letter with text indent
Initial letter with text indent.

The interaction of initial-letters and hanging-punctuation is under discussion.

4.10.3. Ancestor Inlines

If the initial letter box is contained by inline box ancestors, the boundaries of those inline boxes are drawn to exclude the initial letter box, as if it were outside their startmost margin edge. This is a purely geometric operation: it does not affect e.g. property inheritance or the effective letter-spacing between the initial letter box and subsequent content.

4.10.4. Multi-line Initial Letters

If an initial letter is too long to fit on one line, it wraps (according to the usual text-wrapping rules), each line filled and formatted exactly as if it were the first line and the initial letter too long to fit any subsequent normal text. Any normal text after the initial letter starts on its last line, affected exactly as if that line were the first line.

multi-line drop cap
Drop cap extends to two lines.

4.11. Clearing Initial Letters

4.11.1. Raised and sunken caps

The margin box of an initial letter contributes to the size of its containing element. Initial letters that extend above the first line of text, known as “raised caps” or “sunken caps,” do not extend up into previous elements. Since the content box for an initial letter includes all glyph ink, this also means that accents or other ink above the cap height of an initial letter will not impinge on previous elements.

raised cap para after normal para
Raised cap (initial-letters: 3 1) on right; note that the position of the “C” is the same in both cases, but on the right all text is moved down relative to the initial letter.

Handle glyph ink above cap height of font. Proposal: Make it an exclusion area for line boxes and border boxes. Include margin specified on initial-letters as part of exclusion area in order to control spacing.

Draw a box model diagram here. Does the margin of the initial letter collapse with its container?

4.11.2. Short paragraphs with initial letters

A paragraph with an initial letter can have fewer lines of text than the initial letter occupies. In this case, the initial letter’s top alignment is still honored, and its exclusion area continues into any subsequent blocks. This forces the subsequent inline-level content to wrap around the initial letter—exactly as if that block’s text were part of its own containing block. (This is similar to how floats exclude content in subsequent block boxes.)

short para with initial letter
The red text is a short paragraph with an initial letter. Note the subsequent paragraph wraps around the initial letter just as text in the paragraph with the initial letter does.

If the subsequent block starts with an initial letter, establishes an independent formatting context, or specifies clear in the initial letter’s containing block’s start direction, then it must clear the previous block’s initial letter.

short para with initial letter followed by para with initial
The red text is a short paragraph with an initial letter. The subsequent paragraph clears because it also has an initial letter.

4.11.3. Interaction with floats

Initial letters are not floats: they are in-flow inline-level content that belongs to a line box. Therefore:

initial letter interacting with floats
In the absence of an initial letter, the first line of text could abut the blue float. But the presence of the initial letter requires that the text move over.

See CSS2§9.5 for more information about the layout of floats and adjacent content. [CSS2]

Whether an inline-end float originating in subsequent lines must clear the initial letter (as inline-start floats do) is still under discussion. There is no aesthetic reason to require it; however it’s yet unclear how the underlying layout model would distinguish between the two cases.

4.11.4. Interaction with Fragmentation (Pagination)

Since a single glyph cannot be fragmented across pages (or columns or other fragmentation containers), an initial letter is considered monolithic [css-break-3] for the purpose of block-axis fragmentation (breaking across pages, columns, regions, etc.). Additionally, breaks between the in-flow lines alongside an initial letter box are avoided, much as breaks between line boxes affected be widows and orphans are avoided. However, if there is a forced break alongside the initial letter box, then it takes precedence; but has no effect on the initial letter box itself.

As with other monolithic objects, if an initial letter box occurs at the top of a fragmentation container and that fragmentation container is too short to contain it, it may be either truncated or sliced. Adjacent content, however, must be fragmented according to its own rules, not truncated or sliced along with the initial letter.

Appendix A: Synthesizing Alignment Metrics

A.1: Synthesizing Baselines (and Other Font Metrics) for Text

Some fonts might not contain the metrics information necessary to align text properly as described in this module. User agents may use the following strategies in the absence of a required metric:

Measure the font
Metrics may be derived from the glyph shapes. For example,
  1. The center of the minus sign (U+2212) can be taken as the mathematical baseline.
  2. The amount by which the lowercase “o” descends below the alphabetic baseline can be subtracted from its highest point to measure the x-height.
    measuring the x height of the letter o
    Measuring the x height.
  3. The amount by which the uppercase “O” descends below the alphabetic baseline can be subtracted from its highest point to measure the cap-height.
  4. The bounding box of 永 (U+6C38) can be used to find the ideographic character face edges.
  5. The top edge of the center of the Hebrew maqaf (U+05B3 “־”) can be taken as the Hebrew hanging baseline.
  6. The top edge of the center of the letter Ka can be taken as the hanging baseline. Which Ka is used should depend on the content language:
    Language Script Letter
    Devanagari क U+0915 KA
    Bengali ক U+0995
    Gurmukhi ਕ U+0A15
    Tibetan ཀ U+0F40

    Pick a default.

    finding the position of the hanging baseline of the letter ka
    The hanging baseline is at the top edge of the character ink.
  7. Issue: Add more notes here?

Somebody sanity-check these heuristics please.

Use a heuristic for the script
Issue: What does this mean?
Use fallback values
The following fallback values
  • x-height: .5em;
  • cap-height: .66em;
  • hanging baseline: .6em;
  • Hebrew hanging baseline: ???

A.2: Synthesizing Baselines for Replaced Content

Copy over text from CSS Writing Modes and expand for additional baseline values.

Note: Authors can use margins (positive or negative) to adjust the alignment of replaced content within a line.

In this example, the author is using a set of images to display characters that don’t exist.
img[src^="/text/"] {
  height: 1em; /* Size to match adjacent text */
  margin-bottom: -0.2em; /* Baseline at 20% above bottom */
}
...
<p>This is some text with words written in an unencoded script:
<img src="/text/ch3439.png" alt="...">
  <img src="/text/ch3440.png" alt="...">
  <img src="/text/ch3442.png" alt="...">

Note: A future level of CSS may include a way of specifying a full baseline table for replaced elements. (This will probably look like a baseline-table property that accepts ''[<baseline-keyword> <percentage>]+''.)

Changes

Changes since the 24 May 2016 Working Draft include:

Acknowledgments

Special thanks goes to the initial authors, Eric A. Meyer and Michel Suignard.

In additions to the authors, this specification would not have been possible without the help from:

David Baron, David M Brown, Oriol Brufau, John Daggett, Stephen Deach, Sylvain Galineau, David Hyatt, Shinyu Murakami, Tess O’Connor, Sujal Parikh, Florian Rivoal, Alan Stearns, Bobby Tung, Chris Wilson, Grzegorz Zygmunt.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-ALIGN-3]
Elika Etemad; Tab Atkins Jr.. CSS Box Alignment Module Level 3. 30 August 2018. WD. URL: https://www.w3.org/TR/css-align-3/
[CSS-BOX-3]
Elika Etemad. CSS Box Model Module Level 3. 9 August 2018. WD. URL: https://www.w3.org/TR/css-box-3/
[CSS-BREAK-3]
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 3. 9 February 2017. CR. URL: https://www.w3.org/TR/css-break-3/
[CSS-CASCADE-4]
Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. 28 August 2018. CR. URL: https://www.w3.org/TR/css-cascade-4/
[CSS-DISPLAY-3]
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 28 August 2018. CR. URL: https://www.w3.org/TR/css-display-3/
[CSS-FONTS-3]
John Daggett; Myles Maxfield; Chris Lilley. CSS Fonts Module Level 3. 20 September 2018. REC. URL: https://www.w3.org/TR/css-fonts-3/
[CSS-LISTS-3]
Tab Atkins Jr.. CSS Lists and Counters Module Level 3. 20 March 2014. WD. URL: https://www.w3.org/TR/css-lists-3/
[CSS-PSEUDO-4]
Daniel Glazman; Elika Etemad; Alan Stearns. CSS Pseudo-Elements Module Level 4. 7 June 2016. WD. URL: https://www.w3.org/TR/css-pseudo-4/
[CSS-RUBY-1]
Elika Etemad; Koji Ishii. CSS Ruby Layout Module Level 1. 5 August 2014. WD. URL: https://www.w3.org/TR/css-ruby-1/
[CSS-SHAPES-1]
Vincent Hardy; Rossen Atanassov; Alan Stearns. CSS Shapes Module Level 1. 20 March 2014. CR. URL: https://www.w3.org/TR/css-shapes-1/
[CSS-SIZING-3]
Tab Atkins Jr.; Elika Etemad. CSS Intrinsic & Extrinsic Sizing Module Level 3. 4 March 2018. WD. URL: https://www.w3.org/TR/css-sizing-3/
[CSS-TEXT-3]
Elika Etemad; Koji Ishii. CSS Text Module Level 3. 20 September 2018. WD. URL: https://www.w3.org/TR/css-text-3/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 14 August 2018. CR. URL: https://www.w3.org/TR/css-values-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 10 October 2018. WD. URL: https://www.w3.org/TR/css-values-4/
[CSS-WRITING-MODES-4]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 4. 24 May 2018. CR. URL: https://www.w3.org/TR/css-writing-modes-4/
[CSS2]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS2/
[CSS3-EXCLUSIONS]
Rossen Atanassov; Vincent Hardy; Alan Stearns. CSS Exclusions Module Level 1. 15 January 2015. WD. URL: https://www.w3.org/TR/css3-exclusions/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

Informative References

[CSS-LINE-GRID-1]
Elika Etemad; Koji Ishii; Alan Stearns. CSS Line Grid Module Level 1. 16 September 2014. WD. URL: https://www.w3.org/TR/css-line-grid-1/
[CSS-MULTICOL-1]
Florian Rivoal; Rachel Andrew. CSS Multi-column Layout Module Level 1. 28 May 2018. WD. URL: https://www.w3.org/TR/css-multicol-1/
[CSS-PAGE-FLOATS-3]
Johannes Wilm. CSS Page Floats. 15 September 2015. WD. URL: https://www.w3.org/TR/css-page-floats-3/
[CSS-POSITION-3]
Rossen Atanassov; Arron Eicholz. CSS Positioned Layout Module Level 3. 17 May 2016. WD. URL: https://www.w3.org/TR/css-position-3/
[CSS-WRITING-MODES-3]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 3. 24 May 2018. CR. URL: https://www.w3.org/TR/css-writing-modes-3/
[SELECT]
Tantek Çelik; et al. Selectors Level 3. 6 November 2018. REC. URL: https://www.w3.org/TR/selectors-3/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
alignment-baseline baseline | text-bottom | alphabetic | ideographic | middle | central | mathematical | text-top | bottom | center | top baseline inline-level boxes, flex items, grid items, table cells no N/A discrete per grammar specified keyword
baseline-shift <length-percentage> | sub | super 0 inline-level boxes, flex items, grid items no refer to the used value of line-height discrete per grammar the specified keyword and/or a computed <length-percentage> value
dominant-baseline auto | text-bottom | alphabetic | ideographic | middle | central | mathematical | hanging | text-top normal block containers, inline boxes, table rows, table columns, grid containers, and flex containers yes N/A discrete per grammar specified keyword
initial-letters normal | <number> <integer> | <number> && [ drop | raise ]? normal certain inline-level boxes and ::first-letter and inside ::marker boxes (see prose) no N/A by computed value type per grammar the keyword normal or a number paired with an integer
initial-letters-align border-box? [ alphabetic | ideographic | hebrew | hanging ] | border-box alphabetic certain inline-level boxes and ::first-letter and inside ::marker boxes (see prose) yes N/A discrete per grammar specified keyword(s)
initial-letters-wrap none | first | all | grid | <length-percentage> none certain inline-level boxes and ::first-letter and inside ::marker boxes (see prose) yes relative to logical width of (last fragment of) initial letter by computed value type per grammar specified keyword or computed <length-percentage> value
inline-sizing normal | stretch normal inline boxes no n/a discrete per grammar specified keyword
line-height normal | <number> || <length-percentage> normal inline boxes yes computed relative to 1em discrete per grammar the specified keyword, a number, or a computed <length> value
line-sizing quirks-behavior | current-behavior | better-behavior | box-model-behavior | absolute-behavior current-behavior block containers? inline boxes? yes N/A discrete per grammar the specified keyword
vertical-align <‘baseline-shift’> || <‘alignment-baseline’> baseline inline-level boxes no N/A see individual properties per grammar see individual properties

Issues Index

This section is being rewritten. Refer to section 10.8 of [CSS2] for the normative CSS definition or the 2002 Working Draft if you want pretty pictures. (But ignore the old text, half of it’s wrong. We’re not specifying which half, that’s to be determined.) The CSS2 specification should be used as the guideline for implementation.
The CSSWG would like to know which baseline values are necessary: if any can be dropped, or any need to be added. See GitHub issue 859.
Should be text-over and text-under instead of text-top and text-bottom, but maybe it’s better not to use those terms for consistency with legacy vertical-align. See GitHub issue 860.
Add first and last values. Note, in this property, these are combinatorial, whereas in the align/justify-self/content properties, it’s singular. Do we want to align the syntaxes wrt hyphens vs. spaces or what? See GitHub issue 861.
This property will gain first and last keywords, like in the box alignment properties, see CSS Box Alignment 3 §4.2 Baseline Alignment: the baseline keyword and first/last modifiers. The open question is whether they should be added to alignment-baseline or a new sub-property should be created to hold the first | last preference.
Clean up this prose to correctly handle alignment contexts other than inline formatting contexts.
We would prefer to remove this, and are looking for feedback from SVG user agents as to whether it’s necessary.
Fill out this definition so that it fully replaces CSS2.
The fact that percentages compute to lengths is annoying. See also href="https://github.com/w3c/csswg-drafts/issues/3118">Issue 3118 and Issue 2165.
This is a rought draft of a proposal, and has not yet been approved by the CSSWG. Many variants have been included to promote discussion of possible behaviors; not all of them are expected to be kept. See discussion in Issue 3199, Hyatt’s message, and dbaron’s proposal. Also all the keywords are expected to be renamed.
Should this property apply to block containers or to inline boxes? In the latter case, an individual inline could say "pay attention to me" or "don’t pay attention to me".
Need better names. Maybe `loose | normal | strict | absolute`? Maybe `quirks | legacy | normal | absolute`? Something else?
We might want to use this opportunity to more precisely define normal, rename it to match, and possibly introduce any other values that may seem necessary.
The editors would appreciate any examples of drop initials in non-western scripts, especially Arabic and Indic scripts.
Should there be a way to opt out of this behavior? See Github Issue 310.
Renaming this property (and the others in this section) is currently under discussion.
Is there a proper typographic term for the hebrew “hanging” baseline?
What is the proper alignment for South Asian scripts that do not have the explicit hanging baseline, such as Tamil or Telugu? See GitHub issue 864
This only covers the most common cross-linguistic transcription systems. Should we include any other / all script tags in the UA style sheet?
Reword that to handle box-sizing correctly.
What about inheritance to descendants?
Update this calculation to be a) generic across writing systems / alignment points and b) handle non-integer sizes.
Are there other things we need to consider here?
Should the hanging punctuation be included in the box instead (so that the box is drawn around the punctuation when it is made visible through borders/background), but rather only excluded when positioning the box (so that the initial letter remains flush, with the hanging punctuation properly hanging)? See discussion.
Do we need an unconditional first? (I.e. Should we rename this value to auto and add a first value that does not check for spaces?) See GitHub issue 410
This really needs font-relative lengths to be relative to the used size.
These values and related annoyance is likely unnecessary if someone submits a patch to Blink to support first.
Edit figure to show how auto behaves in varying contexts
The interaction of initial-letters and hanging-punctuation is under discussion.
Handle glyph ink above cap height of font. Proposal: Make it an exclusion area for line boxes and border boxes. Include margin specified on initial-letters as part of exclusion area in order to control spacing.
Draw a box model diagram here. Does the margin of the initial letter collapse with its container?
Whether an inline-end float originating in subsequent lines must clear the initial letter (as inline-start floats do) is still under discussion. There is no aesthetic reason to require it; however it’s yet unclear how the underlying layout model would distinguish between the two cases.
Pick a default.
Somebody sanity-check these heuristics please.
Copy over text from CSS Writing Modes and expand for additional baseline values.