CSS Inline Layout Module Level 3

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.

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

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 basline 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 character face under-side baseline to that of its parent.

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 [CSS3-WRITING-MODES] 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

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.

2.2.1. Alignment Point: alignment-baseline longhand

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: baseline-shift longhand

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.

3. Initial Letters

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

3.1. An Introduction to Initial Letters

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.

3.1.1. 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.

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. 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.

The alignment constraints for drop initials depend on the writing system. In ideographic scripts, the initial letter extends from the block-start edge of the first line to the block-end edge of the Nth line.

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

3.1.2. 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.

3.1.3. 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.

3.2. Selecting Initial Letters

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, as defined in [SELECT], can be used to select an initial letter.

Authors who need more control over which characters are included in an initial letter, or who want to apply initial-letter formatting to replaced elements or multiple words an also apply the initial-letter 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-letter: 2; }

3.2.1. Initial Letter Punctuation

Since ::first-letter selects punctuation before or after the first letter, these characters are included in the initial-letter.

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

3.3. Creating Initial Letters: the initial-letter property

This property specifies styling for dropped, raised, and sunken initial letters. It takes the following values:

normal

No special initial-letter 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 argument defines the number of lines the initial letter should sink. Values less than one are invalid. If omitted, it duplicates the first argument, floored to the nearest positive whole number.

Should this be initial-letters (plural) as originally discussed? Since it can apply to multiple letters. See GitHub issue 862.

An initial letter is a box to which initial-letter applies and is not normal: this triggers the special layout considerations described below.

p::first-letter { initial-letter: 2; }

The initial-letter property does not apply to boxes that are not positioned at the start of the line due to bidi reordering.

The initial-letter property is undefined on children of ruby base container boxes and on ruby container boxes.

3.3.1. Properties

The following properties apply to an initial letter:

• all font properties (see [CSS3-FONTS])
• the color and opacity properties (see [CSS3COLOR])
• all background properties (see [CSS3BG])
• any typesetting properties that apply to inline elements (see [CSS3TEXT])
• all text decoration properties (see [CSS3-TEXT-DECOR])
• all margin, border and padding properties (see [CSS2] and [CSS3BG])

Note: Which properties apply to initial letter depend on whether it is an inline box or an initial letter box, as initial letter is a display type.

Add width/height as requested by Tantek... need to integrate with model. See GitHub issue 863

3.3.2. Layout Model

An initial letter is laid out following the steps outlined below:

1. Find the used font size based on its initial-letter size, initial-letter-align alignment points, and font metrics. Note that no layout is required in this step. The font size used for sizing the initial letter contents does not affect its computed font-size property (and therefore has no effect on the computation of em length values, etc.).
2. Size the initial letter’s content box. The content box of a non-atomic inline initial letter is the smallest rectangle required to include the entirety of its glyphs as well as the margin boxes of any atomic inlines it contains.
3. The initial letter’s margin box is placed at the edge of the line. If it has no padding or 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 had initial-letter: none.
4. Exclude content within the inline letter’s margin box according to initial-letter-wrap.

3.3.3. Content-box Size, Margins, Borders, and Padding

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

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

3.4. Alignment of Initial Letters: the initial-letter-align property

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

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

span.initial {
  initial-letter: 2;
  initial-letter-align: ideographic;
}

span.initial {
initial-letter: 2;
initial-letter-align: hebrew;
}

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.

3.4.1. UA Default Stylesheet for initial-letter-align

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

[lang=zh], [lang=ja], [lang=ko], [lang=ii] {
  initial-letter-align: ideographic;
}
[lang|=iw], [lang|=yi], [lang|=lad], [lang|=jrb] {
  initial-letter-align: hebrew;
}
[lang|=hi], [lang|=mr], [lang|=ne], , [lang|=pi], [lang|=kok], [lang|=brx], [lang|=mai], [lang|=sd], [lang|=sa] {
  initial-letter-align: hanging;
}

We don’t have a way to do subtag selection with attr selectors. (It’s a feature of :lang() in Selectors 4, but we don’t want to use that because we want to hit the language root, not all of the descendant elements.) These selectors should also include the relevant script subtags.

3.5. Sizing Initial Letters

The size of a drop initial is determined by the need to satisfy the required alignment. 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:

The line height used in this calculation 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 [CSS3-LINE-GRID]). The contents of the lines spanned, and therefore any variation in their heights and positions, is not accounted for.

3.5.1. Shaping and Glyph Selection

When initial-letter is not normal, shaping does not occur across the box’s boundaries, as if the zero-width non-joiner (U+200C) were inserted before/after the initial letter. For example, if the first letter of the Farsi word “پس” were styled with initial-letter: 2 1, both letters would be styled in their isolated forms, with “پ” as the initial letter, followed by the normally-styled “س”.

Are there other things we need to consider here?

3.6. Space Around 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.

Text is therefore excluded around the glyph bounding boxes of the initial letters.

Specifically, for non-atomic initial letters, the content box of the element is sized to fit:

• The specified amount of sink (i.e the distance from the top alignment point to the bottom alignment point).
• The actual ascent and descent and side bearings of all the glyphs it contains that are part of its inline formatting context, even if they leak outside their em boxes.
• The margin boxes of all the atomic inlines it contains that are part of its inline formatting context, even if they leak outside its own line-height.

The margin box of the initial letter is then made an exclusion area for subsequent text.

3.7. Positioning and Kerning Initial Letters

In the block axis, the initial letter is positioned to satisfy its alignment requirements. (See initial-letter-align.)

In the inline axis, the position of the inline letter is given by aligning its start margin edge to the start edge of the containing block.

However, if the initial letter is a non-atomic inline with zero padding and borders, the UA must apply an additional negative offset on the start side, of the amount necessary to optically align the first glyph to the containing block edge as it would be in normal text.

3.8. Initial Letter Wrapping: the initial-letter-wrap property

Note: initial-letter-wrap is at risk.

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:

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?)

all

For each line of text impacted by the initial letter, the line box adjacent to the intial 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.

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. The justify-self property can then be used to align the initial letter box within the exclusion area.

The rule for matching the character grid is 1ch × n + letter-spacing × (n - 1), where n is the size of the initial letter as specified by the initial-letter property.

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-letter: 3; /* 3-line drop-cap */
  initial-letter-wrap: first;
  margin-right: 0.1em;
}
@supports (not (initial-letter-wrap: first)) {
  /* Classes auto-generated on paragraphs to match first letter. */
  p.A:first-letter {
    initial-letter-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-letter: 3;
  initial-letter-wrap: none;
}

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

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

3.9. Indentation and Multi-line Effects

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.

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.

3.10. Clearing Initial Letters

3.10.1. 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.

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-letter 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?

3.10.2. 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.)

If the subsequent block starts with an initial letter, establishes a new 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.

3.10.3. Interaction with floats

Floats always clear an initial letter when floated to the same side. When floated to the opposite side, they are pushed down to clear the initial letter only if they have specified clear to the affected side or they do not fit without overlapping the initial letter.

Is this the behavior we want? What if the float is on the same line as 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.

   

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 top edge of the below ideograph (U+4E05 “丅”) and the bottom edge of the above ideograph (U+4E04 “丄”) can be taken as the ideographic character face edges. In the horizontal dimension, the left and right edges of the nation ideograph (U+56FD “国”) or reason ideograph (U+56E0 “因”) can be used. In all cases, the these edges should be measured at their intersection with the central baseline in that axis.

   What is the best character to use for the sides?

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.

   

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 17 September 2015 Working Draft include:

• Adding values for the ideographic and middle baselines.
• Clarifying interaction of the auto dominant-baseline value with SVG glyph positioning.
• A few other minor editorial improvements.

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, 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.