CSS Inline Layout Module Level 3

Shortname: css-inline
Level: 3
Status: ED
Group: csswg
TR: http://www.w3.org/TR/css-inline/
ED: http://dev.w3.org/csswg/css-inline/
Previous version: http://www.w3.org/TR/2002/WD-css3-linebox-20020515/
!Issues list: CSS3 Line Layout issues in Bugzilla
Editor: Dave Cramer, Hachette Livre, dauwhe@gmail.com
Editor: Elika J. Etemad, Invited Expert, http://fantasai.inkedblade.net/contact
Editor: Steve Zilles, Adobe, szilles@adobe.com
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 [[CSS3TEXT]].  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]].
Ignored Terms: guru, ji, line-height-shift-adjustment, text-script, after, before

Line box, Line stacking and Content height

This section describes inline boxes formatting, how lines are stacked together and a mechanism to adjust the content height of inline elements. Two terms: inline-progression dimension and block-progression dimension are used frequently in the section and are defined as follows

The inline-progression dimension is a length in the direction of the inline-progression of a box. For an horizontal layout flow it can be perceived as related to the the width of the box. However for a vertical layout flow it will be related to the height of the box. The term inline-progression dimension is the neutral way to express that dimension independently of the layout flow.

The block-progression dimension is a length in the direction of the block-progression of a box. For an horizontal layout flow it can be perceived as related to the height of the box. However for a vertical layout flow it will be related to the width of the box. The term block-progression dimension is the neutral way to express that dimension independently of the layout flow.

Inline formatting context

In an inline formatting context, boxes are laid out in the inline-progression direction, one after the other, following the block-progression within the containing block. Borders, padding and inline-progression margins are respected between these boxes. The boxes may be aligned within the inline-progression in different ways: their under-edges or over-edges may be aligned, or the baselines of text within them may be aligned. The rectangular area that contains the boxes that form a line is called a line box.

The inline-progression dimension ('width' in horizontal flow) of a line box is determined by its containing block. The block-progression dimension ('height' in horizontal flow) of a line box is determined by the rules given in the section on line height calculations. A line box is generally tall (relative) enough for all of the boxes it contains. However, it may be taller than the tallest box it contains (if, for example, boxes are aligned so that baselines line up). The alignment of boxes within a line box is determined by the baseline alignment properties.

In general, when several inline boxes cannot fit within a single line box, they are distributed among two or more block-progression stacked line boxes. The exact wrapping of inline boxes at the line ending edge is determined by several text related property categories such as line-breaking, word-breaking, text wrapping and white-space management. Thus, a paragraph is a stack of line boxes.

Line boxes are stacked together in the block-progression direction without any separation. The block-progression dimension of the line box is determined by the contents of the line box (including the root inline box) and the 'line-box-contain' property.

A containing block defines a root inline box which wraps all the inline children of the block element, including the anonymous inline boxes. It inherits inheritable properties from the parent block box (like 'line-height'), while non-inherited properties have their default values. The root inline box establishes a baseline for vertical alignment and may affect the height of the line boxes.

Although margins, borders, and padding of non-replaced elements do not enter into inline box block-progression dimension calculation (and thus the line box calculation), they are still rendered around inline boxes. This means that if the block-progression dimension of a line box is shorter than the outer edges of the boxes it contains, backgrounds and colors of padding and borders may "bleed" into adjacent line boxes. However, in this case, some user agents may use the line box to "clip" the border and padding areas (i.e., not render them).

Inline-progression: Line Box wrapping

In general, the start edge of a line box touches the start edge of its containing block and the end edge touches the end edge of its containing block. However, floating boxes may come between the containing block edge and the line box edge. Thus, although line boxes in the same inline formatting context generally have the same inline-progression dimension (that of the containing block), they may vary if available inline-progression space is reduced due to floats. Line boxes in the same inline formatting context may vary in block-progression dimension.

When the total inline-progression dimension of the inline boxes on a line is less than the inline-progression dimension of the line box containing them, their inline-progression distribution within the line box is determined by the 'text-align' property. If that property has the value ''justify'', the user agent may stretch the inline boxes as well.

When line-breaking opportunities are available within inline boxes and are not pre-empted by the white-space properties, the inline boxes located at the ending edge of the line box may be split into several boxes and these boxes distributed across several line boxes. When an inline box is split, margins, borders, and padding have no visual effect where the split occurs (or at any split, when they are several). Formatting of margins, borders, and padding may not be fully defined if the split occurs within a bidirectional embedding.

Inline boxes may also be split into several boxes within the same line box due to bidirectional text processing.

<P>Several <EM>emphasized words</EM> appear
<STRONG>in this</STRONG> sentence, dear.</P>

 Several emphasized words appear in this sentence, dear.

Several emphasized words appear
in this sentence, dear.

Several emphasized  
words appear in this 
sentence, dear.

In the previous example, the EM box was split into two EM boxes (call them "split1" and "split2"). Margins, borders, padding, or text decorations have no visible effect after split1 or before split2.

Consider the following example:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
  <HEAD>
    <TITLE>Example of inline flow on several lines</TITLE>
    <STYLE type="text/css">
      EM {
        padding: 2px; 
        margin: 1em;
        border-width: medium;
        border-style: dashed;
        line-height: 2.4em;
      }
    </STYLE>
  </HEAD>
  <BODY>
    <P>Several <EM>emphasized words</EM> appear here.</P>
  </BODY>
</HTML>

Depending on the width of the P, the boxes may be distributed as follows:

   [D]

• The margin is inserted before "emphasized" and after "words".
• The padding is inserted before, above, and below "emphasized" and after, above, and below "words". A dashed border is rendered on three sides in each case.

Block-progression dimensions: the 'text-height' property

Each inline box has a block-progression dimension which is derived from the following parameters: the 'text-height' and 'font-size' properties for non-replaced elements, the height or the width for replaced elements (depending on the text flow being horizontal or vertical respectively) and the stacked block-progression dimension for inline-block elements (identical to what is done for block-level elements).

The block-progression dimension determines the position of the padding, border and margin for the element but does not necessarily determine the line stacking progression as this is also affected by the 'line-box-contain' property.

The 'text-height' property determine the block-progression dimension of the text content area of an inline box (non-replaced elements) . Possible values:

auto

The block-progression dimension is based either on the em square determined by the computed element font-size property value or the cell-height (ascender + descender) related to the computed element font-size as chosen by the user agent.

font-size

The block-progression dimension is based on the em square as determined by the computed element font-size.

text-size

The block-progression dimension is based on the cell-height (ascender + descender) related to the computed element font-size.

max-size

The block-progression dimension is based on the maximum extents toward the over-edge and under-edge of the box obtained by considering all children elements located on the same line, ruby annotations (elements with 'display:ruby-text') and baseline shifted elements.

<number>

The block progression dimension is based on <number> times the em square as determined by the computed font-size.

When more than one font-size is used (this could happen when glyphs are found in different fonts), it is recommended that the largest font-size provides the em square and the cell-height.

Although the 'text-height' property does not apply directly to block elements, it applies to their anonymous children inline boxes (if any).

For replaced elements, the block-progression dimension is determined by the outer edges (i.e. margin edge).

Block progression positioning and spacing

Line height adjustment: the 'line-height' property

The 'line-height' property controls the amount of leading space which is added over and under an inline box (including the root inline box) to determine the extended block-progression dimension of the inline box. The value of the 'line-box-contain' property determines how the block-progression dimension and the extended block-progression dimension affect the height of the line box. The sum of the possible leading space and the block-progression dimension is called the extended block-progression dimension. That extended value is not used for border, margin and padding placement but is used for vertical alignment (relative) and line box block-progression dimension.

The leading for an inline non-replaced element is defined as the difference between the block-progression dimension as determined by the 'text-height' property and the computed value of 'line-height'. Half the leading is called the half-leading, each half-leading is located over and under the element. The 'line-height' value will specify the exact extended block-progression dimension of each box generated by the element. (Depending on the value of 'line-box-contain', the extended block-progression dimension may be ignored.) (For inline replaced elements, both the block-progression dimension and the extended block-progression dimension of the box are given by the outer edges (i.e. margin edge).) Anonymous inline boxes use the 'text-height' and 'line-height' property values specified for their parent.

Depending on the value of the 'line-stacking-strategy', the 'line-height' value may provide a minimum or exact extended block-progression dimension for each generated inline box and the line boxes. How the leading space is distributed between the over-edge and the under-edge of inline elements within the line box depends on their vertical-align property value. [LDB: I don't understand this paragraph.]

Empty inline elements generate empty inline boxes, but these boxes still have a line height, and thus may influence the line box block-progression dimension, as if the empty box contained an infinitely narrow letter.

A inline box which otherwise would influence the line box extended progression dimension may be removed from the computation by using a special value of the line-height property: ''none''.

Values for this property have the following meanings:

normal

Tells user agents to set the computed value to a "reasonable" value based on the font size of the element. The value has the same meaning as <number>. We recommend a computed value for ''normal'' between 1.0 to 1.2. The user agent may allow the <number> to vary depending on the metrics of the font(s) being used.

<length>

The box height is set to this length. Negative values are illegal. The length is the computed value.

<number>

The computed value of the property is this number multiplied by the element's font size. Negative values are illegal. However, the number, not the computed value, is inherited.

<percentage>

The computed value of the property is this percentage multiplied by the element's computed font size. Negative values are illegal.

none

For inline-level elements, the element does not influence the extended block-progression dimension of the line. The computed value is the specified value (''none''). For block-level elements, equivalent to ''normal'' (and the computed value is ''normal''). This definition should be briefer, and should refer to the next section.

DIV { line-height: 1.2; font-size: 10pt }     /* number */
DIV { line-height: 1.2em; font-size: 10pt }   /* length */
DIV { line-height: 120%; font-size: 10pt }    /* percentage */

When an element contains text that is rendered in more than one font, user agents should determine the 'line-height' value according to the largest font size (when appropriate).

Note that replaced elements have a 'font-size' and a 'line-height' property, even if they are not used directly to determine the extended block-progression dimension of the box: ''em'' and ''ex'' values are relative to values of 'font-size' and percentage values for 'vertical-align' are relative to values of the extended block-progression dimension. [LDB: The second part of this sentence is a significant change from CSS2, and also doesn't mauke senes in the context of the sentence.]

When the 'line-height' value is less than the font size, the final inline box extended block-progression dimension will be less than the font size and the rendered glyphs will "bleed" outside the box. If such a box touches the edge of a line box, the rendered glyphs will also "bleed" into the adjacent line box.

Line Stacking: the 'line-box-contain' property

This property enumerates which aspects of the elements in a line box contribute to the height height of that line box.

block

The extended block progression dimension of the root inline box must fit within the line box.

inline

The extended block progression dimension of all non-replaced inline boxes whose 'line-height' is not ''none'' in the line box must fit within the line box.

font

The block progression dimension of all non-replaced inline boxes in the line whose 'line-height' is not ''none'' that directly (i.e., within the box but not within one of its descendants) contain non-removed text must fit within the line box, where non-removed text is any characters not removed based on the values of the 'white-space'. [LDB: This isn't quite right. What about zwnj, etc.?

glyphs

The block progression dimension of all the glyph bounding boxes of glyphs in the line box must fit within the line box.

replaced

The margin box of all replaced elements within the line must fit within the line box.

inline-box

The margin-box of all non-replaced inline elements in the line whose 'line-height' is not ''none'' must fit within the line box. Should this split into inline-border and inline-margin?

If the property has no value for all elements within a line, then the line box has 0 height, and the line within the anonymous inline established by the block that coincides with the line block is the baseline of that anonymous inline block (or should it be determined by the 'vertical-align' property of the block?). [This is arbitrary. Does anyone have better ideas?]

Note that the CSS2 model is equivalent to 'block inline replaced' but the backwards-compatible HTML model is similar to (but not exactly) 'font replaced' [1].

Concerns: * How does this work with the cascade? * What about determining the visual size of inline boxes?

[1] I believe the differences are restricted to the first line of LI elements, the last line of LI, DD, and DT elements, and issues concerning whitespace around images. [DB]

The height of each line box is established as follows (we describe the case for horizontal flow, but vertical flow is analogous). First align all the boxes on the line relative to each other according to the rules for 'vertical-align'. The line box must satisfy all of the following constraints:

1. For each box on the line that has ''font'' in its value of 'line-box-contain', the line box must be high enough to contain the top and bottom of that box's text.
2. For each box that has ''inline'' in 'line-box-contain', the top of the line box must be at least as high as the top of the box's text plus the box's half-leading. The bottom of the line box must be at least as low as the bottom of the text plus the half-leading.
3. For each box that has ''inline-box'' in 'line-box-contain', the top of the line box must be at least as high as the margin-top of this box. The bottom of the line box must be at least as low as the margin-bottom.
4. For each replaced box that has ''replaced'' in its 'line-box-contain', the line box must contain the margin-top and margin-bottom of this box. [The value ''replaced'' is not strictly needed. ''Inline-box'' also does the job, except that it is not automatically restricted to replaced elements.]
5. For each box that has ''glyphs'' in 'line-box-contain', the top of the line box must be at least as high as the top of each glyph in the box (excluding those in child elements). The bottom of the line box must be at least as low as the bottom of each glyph in the box (excluding child elements).
6. If the enclosing block for this line has ''block'' in 'line-box-contain', then the top of the line box must be at least as high as the text top plus the half-leading of an anonymous inline element and the bottom of the line box as low as the text bottom plus the half leading. This must hold whether or not this line actually contains such an element.

Thus ''block'' can be used to set an overall minimum line height (viz. the value of 'line-height' of the element itself) for all lines in an element, independent of the actual contents of each line. In particular, setting 'line-box-contain' to just ''block'' and no other values will ensure that all lines are the same height, at the possible risk of some tall inline elements overlapping with lines above or below.

The half-leading of a box is defined as half the computed value of 'line-height' minus half the computed value of 'font-size', i.e., (line-height - font-size)/2.

The top of the text is the top of the em-box of a box's nominal font, whether or not there actually is any letter that tall. Replaced elements, for example, have no text, but still have a nominal font and are thus a text top. The rules above refer to the position of the text top after the box has been aligned with 'vertical-align'.

Old, remove this:

Line stacking is the mechanism by which a line box is determined for each line in a block and then these lines are stacked in the block-progression direction resolving any spacing constraints between adjacent lines. The line-stacking strategy covers both the determination of the block-progression dimension (height in horizontal flow) of a line box and the rules for spacing line boxes.

All line stacking strategies make use of the properties: 'font-size', 'text-height' and 'line-height'.

The line stacking properties are made of the following properties:

1. The 'line-stacking-strategy' property determines the overall stacking method,
2. The 'line-stacking-ruby' property determines how elements with 'display=ruby' are stacked,
3. The 'line-stacking-shift' property determines how elements which are baseline-shifted are stacked,
4. Finally the 'line-stacking' property is a short-hand for the previous properties.

This property determines the line stacking strategy for stacked line boxes within a containing block element. The term stack-height is used in the context of this property description to indicate the block-progression dimension for the line boxes. Possible values:

inline-line-height

The stack-height is the smallest value that contains the extended block-progression dimension of all the inline elements on that line when those elements are properly aligned. Since the line spacing information is already included in the computation of the line box, these line boxes are simply stacked adjacent to one another in the formatted block to which they belong. The 'line-height' property value is taken into account for both the inline elements and the block elements. For inline elements, it defines the extended block-progression dimension. For block elements, it defines the minimum extended block-progression dimension.

block-line-height

The stack-height is determined by the block element computed 'line-height' property value. The 'line-height' property value is ignored for inline elements. For alignment purpose, this case is similar to the minimum extended block-progression dimension case (strut model). This is the only line-stacking strategy that may cause inline boxes within the line to bleed over and under the line box because the line box is not constrained by its inline boxes. If the block computed 'line-height' is ''none'', the stack-height is determined as if 'line-height: normal'.

max-height

The stack-height is the smallest value that contains the block-progression dimension of all the inline elements on that line when those elements are properly aligned. The 'line-height' property value is taken into account only for the block elements and it defines the minimum extended block-progression dimension.

grid-height

The stack-height is the smallest multiple of the block element 'line-height' computed value that can contain the block-progression of all the inline elements on that line when those elements are properly aligned. The 'line-height' property value is ignored for inline elements. If the block computed 'line-height' is ''none'', the stack-height is determined as if 'line-height: normal'.

When the line-stack strategy dictates that the inline element line-height be ignored, this means that for those elements only their block-progression dimensions are considered for the stack-height, not their extended block-progression dimensions.

Note. XSL has a similar property with the same name which use different but equivalent values: 'line-height' instead of ''inline-line-height'', ''font-height'' instead of ''block-line-height''. It also uses ''max-height''. The value ''grid-height'' is new to the CSS3 property.

 

Name:	line-stacking-ruby
Value:	exclude-ruby | include-ruby
Initial:	exclude-ruby
Applies to:	block-level elements
Inherited:	yes
Percentages:	N/A
Media:	visual
Computed value:	specified value (except for initial and inherit)

This property determines the line stacking method for block elements containing ruby annotation elements (element with 'display: ruby-text' or 'display: ruby-text-container'). In all cases the ruby base elements (elements with 'display: ruby-base' or display: ruby-base-container') are considered for line stacking. Possible values:

exclude-ruby

The ruby annotation elements are ignored for line stacking.

include-ruby

The ruby annotation elements are considered for line stacking.

This property is ignored in two cases:

1. The element text-height property is set to ''max-size''.
2. The line stacking strategy is set to ''block-line-height''.

This property determines the line stacking method for block elements containing elements with baseline-shift. Possible values:

consider-shifts

In determining the stack-height, include the adjusted top-edge and bottom-edge of any characters that have a baseline-shift.

disregard-shifts

In determining the stack-height, include the unshifted top-edge and bottom-edge of any characters that have a baseline-shift.

This property is ignored in two cases:

1. The element text-height property is set to ''max-size''.
2. The line stacking strategy is set to ''block-line-height''.

Note. XSL has a similar property with a different name: 'line-height-shift-adjustment' which use the same values.

Finally, the line stacking strategy can be used using the following shorthand:

Baseline alignment

Baseline alignment describes the alignment of textual content and based on information contained in  font tables associated with font resources. Additional descriptions for these font tables are provided in thea href="http://www.w3.org/Style/Group/css3-src/css3-fonts/Overview.html">CSS3 Fonts module.

Baseline information provided by fonts

The glyphs of a given script are positioned so that a particular point on each glyph, the alignment-point, is aligned with the alignment-points of the other glyphs in that script. The glyphs of different scripts are typically aligned at different points on the glyph. For example, Western glyphs are aligned on the bottoms of the capital letters, certain Indic glyphs (including glyphs from the Devanagari, Gurmukhi and Bengali scripts) are aligned at the top of a horizontal stroke near the top of the glyphs and East Asian glyphs are aligned either at the bottom or center of the EM box of the glyph. Within a script and within a line of text having a single font-size, the sequence of alignment-points defines, in the inline-progression-direction, a geometric line called a baseline. Western and most other alphabetic and syllabic glyphs are aligned to an "alphabetic" baseline, the above Indic glyphs are aligned to a "hanging" baseline and the East Asian glyphs are aligned to an "ideographic" baseline.

A baseline-table specifies the position of one or more baselines in the design space coordinate system. The function of the baseline table is to facilitate the alignment of different scripts with respect to each other when they are mixed on the same text line. Because the desired relative alignments may depend on which script is dominant in a line (or block), there may be a different baseline table for each script. In addition, different alignment positions are needed for horizontal and vertical writing modes. Therefore, the font may have a set of baseline tables: typically, one or more for horizontal writing-modes and zero or more for vertical writing-modes.

The font tables for a font include font characteristics for the individual glyphs in the font. CSS assumes that the font tables include, for each glyph in the font, one width value, one alignment-baseline and one alignment-point for the horizontal writing-modes. If vertical writing-modes are supported, then each glyph must have another width value, alignment-baseline and alignment-point for the vertical writing-modes. (Even though it is specified as a width, for vertical writing-modes the width is used in the vertical direction.)

The script to which a glyph belongs determines an alignment-baseline to which the glyph is to be aligned. The position of this baseline in the design space coordinate system determines the default block-progression direction position of the alignment-point. The inline-progression direction position of the alignment-point is on the start-edge of the glyph.

Baseline identifiers

The baseline alignment properties control the alignment of child element with respect to their parent. The positions of these baselines are illustrated in the following figure:

The baseline-identifiers below are used in this specification. Some of these are determined by baseline-tables contained in a font as described in the section describing the baseline information provided by fonts. Others are computed from other font characteristics as described below. Whether determined by the font or computed, a derived baseline-table is constructed with positions of each of the baselines below.

alphabetic

This identifies the baseline used by most alphabetic and syllabic scripts. These include, but are not limited to, many Western, Southern Indic, Southeast Asian (non-ideographic) scripts.

ideographic

This identifies the baseline used by ideographic scripts. For historical reasons, this baseline is at the bottom of the ideographic EM box and not in the center of the ideographic EM box. See the "central" baseline. The ideographic scripts include Chinese, Japanese, Korean, and Vietnamese Chu Nom.

hanging

This identifies the baseline used by certain Indic scripts. These scripts include Devanagari, Gurmukhi and Bengali.

mathematical

This identifies the baseline used by mathematical symbols.

central

This identifies a computed baseline that is at the center of the EM box. This baseline lies halfway between the text-over-edge and text-under-edge baselines.

Note. For ideographic fonts, this baseline is often used to align the glyphs; it is an alternative to the ideographic baseline.

middle

This identifies a baseline that is offset from the alphabetic baseline in the shift-direction by 1/2 the value of the x-height font characteristic. The position of this baseline may be obtained from the font data or, for fonts that have a font characteristic for "x-height", it may be computed using 1/2 the "x-height". Lacking either of these pieces of information, the position of this baseline may be approximated by the "central" baseline.

text-over-edge

This identifies the over-edge of the EM box. The position of this baseline may be specified in the baseline-table or it may be calculated.

Note. The position of this baseline is normally around or at the top of the ascenders, but it may not encompass all accents that can appear above a glyph. For fonts with ascenders, the value of the "ascent" font characteristic is used. For ideographic fonts, the position of this baseline is normally 1 EM in the shift-direction from the "ideographic" baseline. However, some ideographic fonts have a reduced width in the inline-progression-direction to allow tighter setting. When such a font, designed only for vertical writing-modes, is used in a horizontal writing-mode, the "text-over-edge" baseline may be less than 1 EM from the text-under-edge.

text-under-edge

This identifies the under-edge of the EM box. The position of this baseline may be specified in the baseline-table or it may be calculated.

Note. For fonts with descenders, the position of this baseline is normally around or at the bottom of the descenders. For these fonts the value of the "descent" font characteristic is used. For ideographic fonts, the position of this baseline is normally at the "ideographic" baseline.

There are, in addition, two computed baselines that are only defined for line boxes: over-edge and under-edge. For each line box, there is a dominant-baseline, a baseline-table and a baseline-table font-size which are those of the nearest block-level ancestor element and are applied to its root inline box. Depending on the 'line-stacking-strategy' being used, the line stacking progression (also called stack-height) can be different from the line block-progression dimension. The over-edge and under-edge baselines are always based on the block-progression dimension and are in fact part of its determination.

For an inline element, the extended inline box includes the maximum extent of their static or relative positioned children located in the same line box and may include leading specified by their line-height value. Depending on the line-stacking-strategy value, the extended line box may have a different block-progression dimension. For replaced elements the margin extents are always added.

The following text explains the interaction between the line-stacking-strategy and the various baseline alignment parameters.

• For 'line-stacking-strategy: inline-line-height', all 'line-height' values (set for both inline elements and ancestor block element)  participate on the evaluation of the line block-progression dimension. The position of the dominant-baseline on the start edge of the line box is determined after all vertical (relative) alignments have been performed on the line. The extended inline boxes of all inline elements include the leading introduced by their line-height value.
• For 'line-stacking-strategy: block-line-height', the ancestor block 'line-height' value participates on the evaluation of the line block-progression dimension. The inline element line-height values are ignored. The position of the dominant-baseline on the start edge of the line box is determined by using a virtual zero-width inline box with the block font and line height properties located on the start edge of the line box. The extended inline boxes of all inline elements do not include any extra leading.
• For 'line-stacking-strategy: max-height' and 'line-stacking-strategy: grid-height', only the ancestor block 'line-height' value participates on the evaluation of the line block-progression dimension. The inline element line-height values are ignored. The position of the dominant-baseline on the start edge of the line box is determined after all vertical (relative) alignments have been performed on the line. The extended inline boxes of all inline elements do not include any extra leading.

The "over-edge" and "under-edge" baselines are defined as follows.

over-edge

The offset of the "over-edge" baseline of the line from the dominant-baseline of the line is determined by ignoring all extended inline boxes whose alignment-baseline is either "over-edge" or "under-edge". For the "over-edge", extents are measured from the dominant-baseline in the direction toward the top (relative) of the extended inline box. The "over-edge" baseline offset is set to the maximum extent of the "over-edges" of the extended inline boxes of the remaining areas. If all the extended inline boxes in a line box are aligned either to the "over-edge" or to the "under-edge", then use the offset of the "text-over-edge" baseline of the line as the offset of the "over-edge" baseline of the line.

under-edge

The offset of the "under-edge" baseline of the line from the dominant-baseline of the line is determined by ignoring all extended inline boxes whose alignment-baseline is under-edge. For the "under-edge", extents are measured from the dominant-baseline in the direction toward the bottom (relative) of the extended inline box. The "under-edge" baseline offset is set to the negative of the maximum of (1) the maximum extent of the "under-edges" of the remaining extended inline boxes and (2) the maximum height of the extended inline boxes that are ignored minus the offset of the "over-edge" baseline of the line.

Note. If all the extended inline boxes in a line box are aligned to the "under-edge" then the specification for the "over-edge" will set the "over-edge" baseline to coincide with the "text-under-baseline" of the line. Then, case (2) above will determine an offset to the "bottom-edge" baseline that will align the "over-edge" of the box with the greatest height to its allocation-rectangle to "over-edge" baseline.

Note. The above specifications for "over-edge" and "under-edge" have the following three properties: (1) all extended inline boxes are below the "over-edge", (2) all extended inline boxes are above the "under-edge", and (3) the distance between the "over-edge" and the "under-edge" cannot be decreased without violating (1) or (2). The specified placement of the "over-edge" and "under-edge" is not the only way that (1)-(3) can be satisfied, but it is the only way they can be satisfied with the smallest possible offset to the "over-edge".

For the purpose of over-edge and under-edge baseline computation, inline boxes include the maximum extent of their static or relative positioned children located in the same line box. Maximum alignment extents are measured by including the half leading extent in the appropriate direction for non replaced elements and by using the margin extents for replaced elements.

Examples showing "over-edge" and "under-edge" alignment:

There are also four baselines that are defined only for horizontal writing-modes.

top

This baseline is the same as the "over-edge" baseline in a horizontal writing-mode and is undefined in a vertical writing mode.

text-top

This baseline is the same as the "text-over-edge" baseline in a horizontal writing-mode and is undefined in a vertical writing mode.

bottom

This baseline is the same as the "under-edge" baseline in a horizontal writing-mode and is undefined in a vertical writing mode.

text-bottom

This baseline is the same as the "text-under-edge" baseline in a horizontal writing-mode and is undefined in a vertical writing mode.

Overview of the baseline alignment process

The alignment of an element with respect to its parent is determined by three things: the scaled-baseline-table of the parent and the alignment-baseline and alignment-point of the element being aligned. Prior to alignment, the scaled-baseline-table of the parent may be shifted. The property specifications below provide the information necessary to align the parent and child elements.

There are four properties that control alignment of elements to the above set of baselines: 'dominant-baseline', 'alignment-baseline', 'baseline-shift' and 'alignment-adjust'. These properties are all independent and are designed so that typically only the specification of one of the properties is needed to achieve a particular alignment goal.

The primary baseline alignment property is the 'dominant-baseline' property. This property sets the scaled-baseline-table as a compound value with the three following components:

1. The dominant-baseline-identifier component is the default 'alignment-baseline' to be used when aligning two inline areas.
2. The baseline-table component specifies the positions of the baselines in the font design space coordinates. The baseline-table acts something like a musical staff; it defines particular points along the block-progression-direction to which glyphs and inline elements can be aligned.
3. The baseline-table 'font-size' component is used to scale the positions of the baselines in that baseline table. In a scaled-baseline-table, the positions of the baselines can be adjusted by multiplying the design-space coordinate values by the baseline-table font-size.

Because the value of thea href="/TR/REC-CSS2/fonts.html#font-family-prop">'font-family' property is a list of fonts, to insure a consistent choice of baseline-table we define the nominal font in a font list as the first font in the list for which a glyph data is available. This is the first that could contain a glyph for each character encountered. (For this definition, glyph data is assumed to be present if a font substitution is made or if the font is synthesized.) This definition insures a content independent determination of the font and baseline table that is to be used.

For convenience, the specification will sometimes refer to the baseline identified by the dominant-baseline-identifier component of the "dominant-baseline" property as the "dominant baseline" (in an abuse of terminology).

The model also assumes that each glyph has an 'alignment-baseline' value which specifies the baseline with which the glyph is to be aligned. (The 'alignment-baseline' is called the "Baseline Tag" in the OpenType baseline-table description.) The initial value of the 'alignment-baseline' property uses the baseline identifier associated with the given glyph. Alternate values for 'alignment-baseline' can be useful for glyphs such as a "*" which are ambiguous with respect to script membership.

The model assumes that the font from which the glyph is drawn also has a baseline table, the font baseline-table. This baseline table has offsets in units-per-em from the (0,0) point to each of the baselines the font knows about. In particular, it has the offset from the glyph's (0,0) point to the baseline identified by the 'alignment-baseline'.

The offset values in the baseline-table are in "design units" which means fractional units of the EM. CSS calls thesea href="/TR/REC-CSS2/fonts.html#descdef-units-per-em">"units-per-em". Thus, the current 'font-size' is used to determine the actual offset from the dominant baseline to the alternate baselines.

The glyph is aligned so that its baseline identified by its 'alignment-baseline' is aligned with the baseline with the same name from the dominant baseline-table.

The offset from the dominant baseline of the parent to the baseline identified by the 'alignment-baseline' is computed using the dominant baseline-table and dominant baseline-table font-size. The font baseline-table and font-size applicable to the glyph are used to compute the offset from the identified baseline to the (0,0) point of the glyph. This second offset is subtracted from the first offset to get the position of the (0,0) point in the shift direction. Both offsets are computed by multiplying the baseline value from the baseline-table times the appropriate font-size value.

If the 'alignment-baseline' identifies the dominant baseline, then the first offset is zero and the glyph is aligned with the dominant baseline; otherwise, the glyph is aligned with the chosen alternate baseline.

The third baseline alignment property is the 'baseline-shift' property. Like the properties other than the "dominant-baseline" property, this property does not change the baseline-table or the baseline-table font-size. It does shift the whole baseline table of the parent element so that when an inner inline element is aligned to one of the parents baselines, the position of the inner inline element is shifted.

The fourth alignment property is the 'alignment-adjust' property. This property is primarily used for replaced elements. The "alignment-adjust" property allows the author to assign where, on the start-edge of the object, the alignment point for that element lies.

For alignment purposes, the margins are added to the replaced elements dimensions.

In addition to the following definition of these properties, an informative appendix: B provides usage examples of these properties.

Dominant baseline: the 'dominant-baseline' property

The 'dominant-baseline' property is used to determine or re-determine a scaled-baseline-table. A scaled-baseline-table is a compound value with three components:

• a baseline-identifier for the dominant baseline; it can be qualified as the “dominant baseline-identifier” (or “dominant baseline” in short); the baseline-identifier is basically an 'alignment-baseline' value.
• a derived baseline-table which contains definition for additional baseline-identifiers related to various scripts,
• and a baseline-table font-size.

Note: Although the 'dominant-baseline' property only applies to inline-level and inline-block element, setting it on the block ancestor of an inline element will influence the behavior of that inline element.

Some values of the property re-determine all three values; other only reestablish the baseline-table font-size. Values for the property have the following meaning:

auto

If this property occurs on a block or inline-block element, then the user agent behavior depends on the value of the 'text-script' property. If the value of the 'text-script' property is ''auto'', the ''auto'' value is equivalent to ''alphabetic'' for horizontal 'writing-mode' values and ''central'' for vertical 'writing-mode' values. If the value of the 'text-script' property is other than ''auto'', the ''auto'' value is equivalent to ''use-script''.

Otherwise, if this property occurs on an inline-level element, then the baseline-identifier and the baseline-table components remain the same as those of the parent element. The baseline-table font-size also remains the same as the parent's one, unless the computed 'baseline-shift' value actually shifts the baseline; then the baseline-table font-size is set to the value of the 'font-size' property on this element. If there is no parent element, the dominant-baseline components are set as for the block elements.

use-script

The dominant baseline-identifier is set using the computed value of the 'text-script' property. If the 'text-script' is set to ''auto'', the dominant script established by that property is used (see the description of 'text-script' in the CSS3 text module for more details). The 'writing-mode' value, whether horizontal or vertical is used to select the baseline-table that correspond to that baseline-identifier. The baseline-table font-size component is set to the value of the 'font-size' property on this element.

no-change

The dominant baseline-identifier, the baseline-table and the baseline-table font-size remain the same as that of the parent.

reset-size

The dominant baseline-identifier and the baseline table remain the same, but the baseline-table font-size is changed to the value of thea href="/TR/REC-CSS2/fonts.html#font-size-props">'font-size' property on this element. This re-scales the baseline table for the current 'font-size'.

alphabetic

The dominant baseline-identifier is set to the ''alphabetic'' baseline, the derived baseline-table is constructed using the ''alphabetic'' baseline-table in the nominal font, and the baseline-table font-size is changed to the value of the 'font-size' property on this element. (The ''alphabetic'' baseline is the standard baseline for Roman scripts.)

hanging

The dominant baseline-identifier is set to the ''hanging'' baseline, the derived baseline- table is constructed using the ''hanging'' baseline-table in the nominal font, and the baseline-table font-size is changed to the value of the 'font-size' property on this element.

ideographic

The dominant baseline-identifier is set to the ''ideographic'' baseline, the derived baseline- table is constructed using the ''ideographic'' baseline-table in the nominal font, and the baseline-table font-size is changed to the value of the 'font-size' property on this element.

mathematical

The dominant baseline-identifier is set to the ''mathematical'' baseline, the derived baseline- table is constructed using the ''mathematical'' baseline-table in the nominal font, and the baseline-table font-size is changed to the value of the 'font-size' property on this element.

central

The dominant baseline-identifier is set to be ''central''. The derived baseline-table is constructed from the defined baselines in a baseline-table in the nominal font. That font baseline-table is chosen using the following priority order of baseline-table names: ''ideographic'', ''alphabetic'', ''hanging'' and ''mathematical''. The baseline-table is changed to the value of the 'font-size' property on this element.

middle

The dominant baseline-identifier is set to be ''middle''. The derived baseline-table is constructed from the defined baselines in a baseline-table in the nominal font. That font baseline-table is chosen using the following priority order of baseline-table names: ''alphabetic'', ''ideographic'', ''hanging'' and ''mathematical''. The baseline-table is changed to the value of the 'font-size' property on this element.

text-under-edge

The dominant baseline-identifier is set to be ''text-under-edge''. The derived baseline-table is constructed from the defined baselines in a baseline-table in the nominal font. That font baseline-table is chosen using the following priority order of baseline-table names: ''alphabetic'', ''ideographic'', ''hanging'' and ''mathematical''. The baseline-table is changed to the value of the 'font-size' property on this element.

text-over-edge

The dominant baseline-identifier is set to be ''text-over-edge''. The derived baseline-table is constructed from the defined baselines in a baseline-table in the nominal font. That font baseline-table is chosen using the following priority order of baseline-table names: ''alphabetic'', ''ideographic'', ''hanging'' and ''mathematical''. The baseline-table is changed to the value of the 'font-size' property on this element.

Note: Computed baselines do not (necessarily) choose a defining baseline table from the nominal font. The expected baseline-tables that a font would have are those that correspond to the defined baselines. The catch is that the positions of the defined baselines -- the ''hanging'', ''alphabetic'', ''mathematical'' and ''ideographic'' baselines -- can be specified differently for different choices of which of these baselines is dominant. That is, the absolute value of the offset of an alphabetic baseline from the ideographic baseline in ideographic text may be different from the offset of the ideographic baseline from the alphabetic baseline in alphabetic text. Therefore, it matters which baseline-table from the nominal font is used for the defined baselines.

If there is no baseline-table in the nominal font or if the baseline-table lacks an entry for the desired baseline, then the user agent may use heuristics to determine the position of the desired baseline.

Aligning the alignment point of an element: the 'alignment-baseline' property

This property specifies how an inline-level element is aligned with respect to its parent. That is, to which of the parent's baselines the alignment point of this element is aligned. Unlike the 'dominant-baseline' property the 'alignment-baseline' property has no effect on its children dominant-baselines.

Note: The 'alignment-adjust' property specifies how the alignment point is determined and defaults to the baseline with the same name as the computed value of the alignment-baseline property.

Except for ''use-script'', all baseline values refer to the respective baseline-identifier components of the dominant-baseline of the parent, and glyphs within the element are aligned similarly to the element itself. The description for ''use-script'' covers these points specifically. The property values have the following meanings:

baseline

The alignment-point of the element being aligned is aligned with the dominant baseline of the parent.

use-script

If the element 'text-script' property value is ''auto'', the alignment point of each glyph is aligned with the parent baseline-identifier of the script to which the glyph belongs. If the element 'text-script' property value is other than ''auto'', the alignment point of each glyph is aligned with the parent baseline-identifier of the script specified by the 'text-script' property. The parent baseline-identifier position is determined by using the baseline information appropriate to the determined script within the parent dominant-baseline set.

over-edge

The alignment point of the box is aligned with the over-edge baseline of the line box.

text-over-edge

The alignment-point of the element being aligned is aligned with the ''text-over-edge'' baseline of the parent.

under-edge

The alignment point of the box is aligned with the under-edge baseline of the line box.

text-under-edge

The alignment-point of the element being aligned is aligned with the ''text-under-edge'' baseline of the parent.

central

The alignment point of the box is aligned with the ''central'' baseline of the parent.

middle

The alignment point of the box is aligned with the ''middle'' baseline of the parent.

ideographic

The alignment-point of the element being aligned is aligned with the ''ideographic'' baseline of the parent.

alphabetic

The alignment-point of the element being aligned is aligned with the alphabetic baseline of the parent.

hanging

The alignment-point of the element being aligned is aligned with the hanging baseline of the parent.

mathematical

The alignment-point of the element being aligned is aligned with the mathematical baseline of the parent.

The values: over-edge, text-over-edge, under-edge and text-under-edge all work relatively to the writing-mode property values. For example over-edge means 'top' in an horizontal writing mode and 'right' in a vertical writing mode.

Note. The reason why ''baseline'' is the initial value instead of ''use-script'' (called ''auto'' in the similar XSL property) has to do with the fact that most fonts today are designed with an alignment point located at the ''alphabetic'' level, even for glyphs belonging to non Latin scripts. User agents have to deal with that constraint, and therefore they use the ''baseline'' value as initial.

Setting the alignment point: the 'alignment-adjust' property

The 'alignment-adjust' property allows more precise alignment of elements, such as graphics, that do not have a baseline-table or lack the desired baseline in their baseline-table. With the 'alignment-adjust' property, the position of the baseline identified by the 'alignment-baseline' can be explicitly determined. It also determines precisely the alignment point for each glyph within a textual element. The user agent should use heuristics to determine the position of a non existing baseline for a given element.

Values for the property have the following meaning:

auto

For each glyph corresponding to textual information within the element, the alignment-point is the intersection of the start-edge of the glyph box and the block-progression-direction position of the alignment point from the font. Padding, border or margin do not affect that alignment point. The alignment point of the inline-level element itself is at the intersection of the start-edge of the first inline box and the baseline identified by the 'alignment-baseline' property if this baseline exists in the baseline-table for the element dominant-baseline. If that specific baseline does not exist, the user agent may use heuristics to determine where that missing baseline would be. For other inline box content like images, the user agent will use heuristics to determine the position of the alignment point. For example when the resulting baseline is ''alphabetic'' or ''ideographic'', it is expected that the alignment point will be at the intersection of the start-edge and the under-edge of the inline box, including its respective margin. If the resulting baseline is ''hanging'', the intersection of the start-edge and the over-edge of the inline box, including its respective margin should be used instead.

When the 'alignment-baseline' property is set to either under-edge or over-edge, the 'alignment-adjust: auto' value is equivalent to under-edge or over-edge respectively.

baseline

The alignment point is at the intersection of the start-edge of the element and the dominant-baseline of the element.

over-edge

The alignment point is at the intersection of the start-edge of the element and the over-edge of the extended inline box of the element. This may include or not the line-height of the element, depending on the line-stacking-strategy.

text-over-edge

The alignment point is at the intersection of the start-edge of the element and the ''text-over-edge'' baseline of the element.

central

The alignment point is at the intersection of the start-edge of the element and the ''central'' baseline of the element.

middle

The alignment point is at the intersection of the start-edge of the element and the ''middle'' baseline of the element.

under-edge

The alignment point is at the intersection of the start-edge of the element and the under-edge of the extended inline box of the element. This may include or not the line-height of the element, depending on the line-stacking-strategy.

text-under-edge

The alignment point is at the intersection of the start-edge of the element and the ''text-under-edge'' baseline of the element.

ideographic

The alignment point is at the intersection of the start-edge of the element and the ''ideographic'' baseline of the element.

alphabetic

The alignment point is at the intersection of the start-edge of the element and the ''alphabetic'' baseline of the element.

hanging

The alignment point is at the intersection of the start-edge of the element and the ''hanging'' baseline of the element.

mathematical

The alignment point is at the intersection of the start-edge of the element and the ''mathematical'' baseline of the element.

<percentage>

The computed value of the property is this percentage multiplied by the computed 'line-height' of the element. The alignment point is on the start-edge of the inline box. Its position along the start-edge relative to the intersection of the dominant-baseline and the start-edge is offset by the computed value. The offset is opposite to the shift-direction (positive value) or in the shift-direction (negative value). A value of ''0%'' makes the dominant-baseline the alignment point.

<length>

The alignment-point is on the start-edge of the inline box. Its position along the start-edge relative to the intersection of the dominant-baseline and the start-edge is offset by the <length> value. The offset is opposite to the shift-direction (positive value) or in the shift-direction (negative value). A value of ''0cm'' makes the dominant-baseline the alignment point.

Repositioning the dominant baseline: the 'baseline-shift' property

The 'baseline-shift' property allows repositioning of the dominant-baseline relative to the dominant-baseline. The shifted object might be a sub- or superscript. Within the shifted element, the whole baseline table is offset; not just a single baseline. For sub- and superscript, the amount of offset is determined from the nominal font of the parent.

Values for the property have the following meaning:

baseline

There is no baseline shift; the dominant baseline remains in its original position.

sub

The dominant baseline is shifted to the default position for subscripts. The offset for this position is determined by the font data for the parent nominal font as adjusted by the dominant baseline-table font-size of the parent element. If there is no applicable font data the User Agent may use heuristics to determine the offset.

super

The dominant baseline is shifted to the default position for superscripts. The offset for this position is determined by the font data for the parent nominal font as adjusted by the dominant baseline-table font-size of the parent element. If there is no applicable font data the User Agent may use heuristics to determine the offset.

<percentage>

The computed value of the property is this percentage multiplied by the computed 'line-height' of the parent element. The dominant-baseline is shifted in the shift-direction (positive value) or opposite to the shift-direction (negative value) of the parent area by the computed value. A value of ''0%'' is equivalent to ''baseline''.

<length>

The dominant-baseline is shifted in the shift-direction (positive value) or opposite to the shift-direction (negative value) of the parent area by the <length> value. A value of ''0cm'' is equivalent to ''baseline''.

Note. Although it may seem that 'baseline-shift' and 'alignment-adjust' properties are doing the same thing, there are important differences. For 'alignment-adjust' the percentage values refer to the 'line-height' of the element being aligned. For 'baseline-shift the percentage values refer to the 'line-height' of the parent element. Similarly, it is the ''sub'' and ''super'' offsets of the parent that are used to align the shifted baseline rather than the ''sub'' and ''super'' offsets of the element being positioned. To ensure a consistent sub- or superscript position, it makes more sense to use the parent as the reference rather than the subscript element which may have a changed "line-height" due to "font-size" changes in the sub- or superscript element.
Using the "alignment-adjust" property is more suitable for positioning elements, such as graphics, that have no internal textual structure. Using the "baseline-shift" property is intended for sub- and superscripts where the positioned element may itself be textual. The baseline-shift provides a way to define a specific baseline offset other than the named offsets that are defined relative to the dominant-baseline. In addition, having "baseline-shift" makes it easier for a tool to generate the relevant properties; many formatting programs already have a notion of baseline shift.

Vertical alignment: the 'vertical-align' shorthand baseline alignment property

This property affects the vertical positioning of the inline boxes generated by an inline-level element inside a line box. The following values only have meaning with respect to a parent inline-level element, or to a parent block-level element, if that element generatesa href="/TR/REC-CSS2/visuren.html#anonymous">anonymous inline boxes; they have no effect if no such parent exists.

auto

Align the dominant baseline of the parent box with the equivalent, or heuristically reconstructed, baseline of the element inline box. If the element doesn't have a baseline, align the bottom of the inline box, including its margin with the parent's dominant baseline. If there is no parent or if there is a change of flow orientation between this element and its parent, the dominant baseline is set to ''alphabetic'' for horizontal flow and ''central'' for vertical flow.

baseline

Align the ''alphabetic'' baseline of the element with the ''alphabetic'' baseline of the parent element. If the inline element doesn't have an ''alphabetic'' baseline, align the bottom of the box, including its margin for replaced elements, with the parent's ''alphabetic'' baseline. The dominant baseline is set to ''alphabetic'' if there is no parent or if there is a flow orientation change between this element and its parent, otherwise it is set to ''no-change''.

central

Align the ''central'' baseline of the inline element with the central baseline of the parent.

middle

Align the ''middle'' baseline of the inline element with the middle baseline of the parent.

sub

Lower the baseline of the box to the proper position for subscripts of the parent's box. (This value has no effect on the font size of the element's text.)

super

Raise the baseline of the box to the proper position for superscripts of the parent's box. (This value has no effect on the font size of the element's text.)

text-top

Align the top of the box with the over-edge of the parent element's font.

text-bottom

Align the bottom of the box with the under-edge of the parent element's font.

<percentage>

Raise (positive value) or lower (negative value) the box by this distance (a percentage of the computed 'line-height' of the element). The value ''0%'' means the same as ''baseline''.

<length>

Raise (positive value) or lower (negative value) the box by this distance. The value ''0cm'' means the same as ''baseline''.

The remaining values refer to the line box in which the generated box appears:

top

Align the over edge of the extended inline box with the over-edge of the line box.

bottom

Align the under edge of the extended inline box with the under-edge of the line box.

The 'vertical-align' property values affect the baseline alignment properties for which it is a shorthand in the following manner:

vertical-align value	alignment-baseline	alignment-adjust	baseline-shift	dominant-baseline
auto	baseline	auto	baseline	auto
baseline	alphabetic	auto	baseline	auto
sub	baseline	auto	sub	auto
super	baseline	auto	super	auto
top	over-edge	auto	baseline	auto
text-top	text-over-edge	auto	baseline	auto
central	central	auto	baseline	auto
middle	middle	auto	baseline	auto
bottom	under-edge	auto	baseline	auto
text-bottom	text-under-edge	auto	baseline	auto
<percentage>	baseline	<percentage>	baseline	auto
<length>	baseline	<length>	baseline	auto

Inline box alignment: the 'inline-box-align' property

The 'inline-box-align' property determines which line of a multi-line inline block aligns with the previous and next inline elements within a line. The alignment strategy for the inline block itself (i.e. the definition of it alignment point and which parent baseline should be used for the alignment) is determined by the inline block element baseline alignment properties applicable to the line being used for the alignment. This property has no effect for single line inline block. Possible values:

initial

Use the initial line of the inline block element for alignment purpose.

last

Use the last line of the inline block element for alignment purpose.

<integer>

Use nth line (as determined by the integer value) of the inline block element for alignment purpose.

Initial Lines and Initial Letters

Initial Lines

• font properties
• color properties
• background properties
• text decoration properties
• baseline alignment properties
• 'word-spacing'
• 'letter-spacing'
• 'line-height' (may be ignored depending on the block line-stacking strategy)
• 'text-shadow'
• 'clear'

An Introduction to Initial Letters

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.

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

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:

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).

Input from Japanese typographers would be very helpful.

Two-line drop initial in vertical writing mode

This figure is probably incorrect.

Add content describing constraint that the block container have a baseline grid (or fixed line-height?).

Selecting Initial Letters

• font properties
• color properties
• background properties
• text decoration properties
• baseline alignment properties (only if 'float' is ''none'')
• 'text-transform'
• 'letter-spacing'
• 'line-height' (may be ignored depending on the block line-stacking strategy)
• margin properties
• padding properties
• border properties
• 'float'
• text-shadow
• 'clear'
• 'initial-letter'
• 'initial-letter-align'

Creating Initial Letters: The 'initial-letter' Property

Name: initial-letter
Value: [<> <>?]
Initial: normal
Applies to: ::first-letter pseudo elements and inline level first child of a block container
Inherited: no
Percentages: N/A
Media: visual
Computed value: ???

Three-line drop initial.

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

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

p::first-letter {
  initial-letter: 2 3;
}

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

p::first-letter {
  initial-letter: 1 3;
}

Alignment of Initial Letters

Name: initial-letter-align
Value: [ auto | alphabetic | hanging | ideographic ]
Initial: auto
Applies to: ::first-letter pseudo elements and inline level first child of a block container
Inherited: no
Percentages: N/A
Media: visual
Computed value: ???

auto

The user agent selects the value which corresponds to the language of the text. Western languages would default to ''alphabetic'', CJK languages to ''ideographic'', and some Indic languages to ''hanging''.

alphabetic

As described above, the cap height of the initial letter aligns with the cap height of the first line of text. The baseline of the initial letter aligns with the baseline of the Nth text baseline.

hanging

The hanging baseline of the initial letter aligns with the hanging baseline of the first line of text.

ideographic

The initial letter is centered in the N-line area.

Input from those knowledgeable about non-Western typographic traditions would be very helpful in describing the appropriate alignments. More values may be required for this property.

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

Space Around Initial Letters

Incorrect: three-line initial letter with descender

Correct: text excluded around glyph bounding box

When the initial letter has a descender, we can say that the other text should be excluded around the em-square. But for characters without descenders, this is undesirable. How do we spec this?

Add content about what happens when initial letter extends into subsequent paragraphs, where the subsequent paragraph may or may not have initial letters.

Define appropriate behavior for non-western scripts. The editors would appreciate any examples of drop initials in such scripts.

Properties index

In addition to the specified values, all properties take the initial and inherit values.

The following properties are mentioned in this module, but are defined elsewhere:

• text-script, direction, text-align, writing-mode, see the Text module.
• font-size, see the Fonts module.

Profiles

There are 3 modules defined by this chapter:

CSS1 line model:

CSS2 line model:

CSS3 line model:

The CSS1 line module is made of the following properties/values:

Name	Values	Initial	Applies to	Inherited	Percentages	Media groups
'line-height'	normal | <number> | <length> | <percentage>	normal	all elements	yes	relative to the font-size of the element itself	N/A
'vertical-align'	baseline | sub | super | top | text-top | middle | bottom | text-bottom | <percentage>	baseline	inline-level elements	no	N/A	N/A

The following table describes the CSS2 text module. Because all properties have added the ''inherit'' value and have a media type, all CSS1 properties have been specified below as well. In addition, the 'vertical-align' has a new value: <length>. Properties that applies to all elements also applies to generated content. Finally, 'vertical-align' also applies to table-cell elements.

.Name	Values	Initial	Applies to	Inherited	Percentages	Media groups
'line-height'	normal | <number> | <length> | <percentage>	normal	all elements and generated content	yes	relative to the font-size of the element itself	N/A
'vertical-align'	baseline | sub | super | top | text-top | central | middle | bottom | text-bottom |a href="/TR/REC-CSS2/syndata.html#percentage-units"><percentage> |a href="/TR/REC-CSS2/syndata.html#length-units"><length> | inherit	baseline	inline-level and table-cell elements	no	N/A	visual

The CSS3 module adds the following properties:

• Line stacking and content height: 'line-stacking-strategy', 'line-stacking-ruby', 'line-stacking-shift', 'line-stacking', 'text-height'
• Baseline alignment: 'alignment-adjust', 'alignment-baseline', 'baseline-shift', 'dominant-baseline', 'inline-box-align'
• Drop initial letter: 'initial-letter', 'initial-letter-align'

It also modifies the following properties as described:

• 'vertical-align' has one new value: ''auto'', and has its initial value changed to ''auto''.
• all properties take the value ''initial''.

Appendix A: Usage of baseline alignment (informative)

The following appendix shows some examples of baseline alignment, exercising the related properties .

A simple example of alignment is shown in the following figure. The figure shows the presentation of two inline elements, one inside the other. These inline elements make up the content of a line in a block where the writing-mode is "lr-tb" and the font is "Helvetica". The structure of the example is as follows:

<p><span class="outer">Apex <span class="inner">Top</span></span></p>

with the style being defined as:

p { writing-mode: lr-tb; font: Helvetica; }
span { alignment-baseline: use-script; }

The other baseline alignment initial values apply. Since a horizontal writing-mode is in use, the dominant-baseline-identifier is set to "alphabetic" and the baseline-table is taken from the nominal-font for the block in which the line appears, which, in this case, is Helvetica.

In the figure, the positions of the baselines relative to the current font size are shown as red (staff) lines. These lines are labeled with abbreviations of the names of the baselines (e.g., TBE for "text-over-edge"). The baseline identified by the dominant-baseline-identifier (A) is shown in blue. There is a break in the staff lines to separately show the inner inline element. This is not necessary for this example, but this distinction will become important in subsequent examples.

The "alignment-baseline" property is the primary control on the positioning of an inner element with respect to its parent. The initial value of the "alignment-baseline" property is "baseline". This aligns the dominant-baseline of the inner inline element with the dominant baseline of the outer inline element. This is shown by the short blue line that connects the two separated staffs (A) in the figure.

The glyphs that are in the content of the two elements are aligned based on the script to which the glyph belongs. Since this example only has Latin glyphs, they are aligned to the "alphabetic" baseline.

In the next figure, the content of the inner inline element is in Gurmukhi, the script of the Punjabi language. The Gurmukhi syllables are read as “guru”. Rather than use Unicode values for these characters, they are symbolized by placing the Latin transliteration in italic type. The structure of the example becomes (assuming the same style):

<p><span class="outer">Apex <span class="inner">guru</span></span></p>

The only change from the previous example is that the glyphs of the Gurmukhi script are aligned to the "hanging" baseline of the inner inline element. The alignment of that element itself, with respect to the outer inline element, is unchanged. The "hanging" baseline position is computed from the font-table part of the dominant-baseline set of the parent element (in this case the outer inline element which is getting itself the set from its parent, the block element).

In the next figure, fragments of the text of the previous examples make up the content of the outer inline element. The inner inline element has a change of font-size, however. The structure is:

<p><span class="outer">Apguru <span class="inner">Exji</span></span></p>

with the following style:

p { writing-mode: lr-tb; font: Helvetica; }
span { alignment-baseline: use-script; }
span.inner { font-size: .75em; }

In this example, the alignment of the inner inline element itself does not change, nor does the alignment of the smaller glyphs inside the inner element. The Latin glyphs are still aligned to the "alphabetic" baseline and the Gurmukhi glyphs, which are pronounced "ji" are aligned to the "hanging" baseline. Note also that just changing the "font-size" property did not change the baseline-table in effect in the inner inline element.

The next figure is equivalent to the previous example with the Gurmukhi character replaced by ideographic characters. These are aligned to the "ideographic" baseline.

To change the scaling of the lines of the baseline table, it is necessary to use the "dominant-baseline" property on the inner inline element. The value of "reset-size" causes the baseline-table font-size to be reset from the font-size of the element on which the "dominant-baseline" property appears. The next figure shows the effect of this, using the structure:

<p><span class="outer">Apguru <span class="inner">Exji</span></span></p>

with the following style:

p { writing-mode: lr-tb; font: Helvetica; }
span { alignment-baseline: use-script; }
span.inner { font-size: .75em; dominant-baseline: reset-size; }

The alignment of the inner inline element, with respect to the outer inline element, is still determined by aligning the dominant baselines (alphabetic). But, the baseline-table of the inner inline element has been rescaled to the font-size of the inner inline element. Hence the smaller glyphs align with each other.

But, what if it is more important that the small Gurmukhi glyphs align with the large Gurmukhi glyphs than having the Latin glyphs align. There are at least two ways to achieve this. The structure:

<p><span class="outer">Apguru <span class="inner">Exji</span></span></p>

with the following style:

p { writing-mode: lr-tb; font: Helvetica; }
span { alignment-baseline: use-script; }
span.outer {dominant-baseline: hanging }
span.inner { font-size: .75em; dominant-baseline: reset-size; }

is illustrated in the next figure. The "hanging" baseline becomes the dominant baseline and the initial value of the "alignment-baseline" property causes the (newly) dominant "hanging" baselines to be aligned as is shown by the connection of the blue baselines.

It is also possible to achieve the effect of the above figure without changing the dominant baseline. Instead it is sufficient to explicitly specify that the inner inline element is aligned on its "hanging" baseline. This is done by:

<p><span class="outer">Apguru <span class="inner">Exji</span></span></p>

with the following style:

p { writing-mode: lr-tb; font: Helvetica; }
span { alignment-baseline: use-script; }
span.inner { font-size: .75em; dominant-baseline: reset-size; alignment-baseline: hanging; }

The only change this approach would make in the above figure is to color the "hanging" baseline red and keep the "alphabetic" baseline as the (blue) dominant baseline. This baseline in the inner inline element would not (as it does not in the above figure) align with the "alphabetic" baseline in the outer inline element.

Another baseline alignment property is the "baseline-shift" property. Like the properties other than the "dominant-baseline" property, this property does not change the baseline-table or the baseline-table font-size. It does shift the whole baseline table of the parent element so that when an inner inline element is aligned to one of the parents baselines, the position of the inner inline element is shifted. This is illustrated in the next figure. The structure which creates this figure is:

<p><span class="outer">Apex <span class="inner">1ji</span></span></p>

with the following style:

p { writing-mode: lr-tb; font: Helvetica; }
span { alignment-baseline: use-script; }
span.inner { baseline-shift: super; }

Because the whole set of baseline-table staff lines are shifted to the position of the superscript baseline: it does not matter to which baseline the glyphs in the superscript are aligned. The European number "1" is aligned to the "alphabetic" baseline and the Gurmukhi syllable "ji" is aligned to the "hanging" baseline.

It is more common for the font-size of the superscript text to be smaller than the font-size of the text to which it is appended. Consider:

<p><span class="outer">Apex <span class="inner">1ji</span></span></p>

with the following style:

p { writing-mode: lr-tb; font: Helvetica; }
span { alignment-baseline: use-script; }
span.inner { baseline-shift: super; font-size: .75em; }

Because changing the font-size on a superscript (or subscript) is common, this is the one case where changing the font-size does cause the baseline-table font-size to be reset when the "dominant-baseline" property has its initial value. After the rescaling, the default alignment to the dominant baseline positions the inline element for the superscript to the dominant baseline position in the shifted baseline-table of the parent element.

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, John Daggett, Stephen Deach, Sujal Parikh, Grzegorz Zygmunt, Chris Wilson, David M Brown.