CSS Line Layout Module Level 3

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

Status of this document

Table of Contents

1 Initial Lines and Drop Initials

1.1 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

Note that the text-indent property typically only affects the initial line of a block element, unless its value is set to hanging.

1.2 An Introduction to Drop Initials

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.

A drop initial 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 occuppies. Two- and three-line drop initial are very common.

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.

Drop initials are all about alignment. Reference points on the drop cap must align precisely with reference points in the text. In the most common case, used in western languages, the top reference points are the cap height of the drop initial and of the first line of text. The bottom reference points are the baseline of the drop initial, and the baseline of the Nth line of text. Figure three shows a simple two-line drop cap, with the relevant reference lines marked.

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

Letters with descenders cause problems. This font is Minion Pro.

cD = (N-1) * L + cF

1.3 Selecting Drop Initials

Should we just skip this, and say that if an element has the dorp

• 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
• all properties described in this section (drop-initial-*)

1.4 Sizing Drop Initials: the drop-initial-value and drop-initial-size Properties

The drop-initial-value both creates a drop initial (if its value is an integer larger than 1), and determines which line of text the after edge of the drop initial aligns to. If the value is 1, no drop initial is created.

Three-line drop initial.

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

Some styles of drop initials do not align with the first line of text. For example, “sunken caps” both sink below the first baseline, and extend above the first line of text. In these cases, the size of the initial cap needs to be defined. The drop-initial-size property defaults to auto, which produces an auto-sized drop cap. But lengths and percentages can also be used, but doing so overrides any constraints on the before-edge of the drop cap.

Sunken drop initial, with size matching a three-line drop initial.

p::first-letter {
drop-initial-value: 2;
drop-initial-size: 52pt;
}

Possible values:

auto

The drop initial letter is sized according to the constraints created by the ink filling strategy and the two connection lines.

As noted elsewhere, this strategy may be untenable.

<integer>

The drop initial letter is sized using the combined line height of the nth lines (as determined by the line value). The letter may be stretched on one dimension if the line height of each line is variable to avoid circular issues.

What use case does this address that isn’t handled by simply setting the size to a length? This could be confusing for authors, since drop-initial-size: 3 does not give you a 3-line drop cap.

<length>

The drop initial letter is sized using the length value.

Perhaps clarify that if a physical length is given, that is used as the font-size of the drop initial, rather than the physical height of the letter?

<percentage>

The drop initial letter is sized relatively to the combined line height of the n lines determined by the drop-initial-value property value.

Again, will the result be a font-size, or will it be measuring the height of the letter?

Acknowledgments

Thanks to John Daggett, Liam Quin and James Clark for invaluable feedback.

Conformance

Document conventions

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

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

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

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

Note, this is an informative note.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet

A CSS style sheet.

renderer

A UA that interprets the semantics of a style sheet and renders documents that use them.

authoring tool

A UA that writes a style sheet.

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

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

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

Partial implementations

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

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

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

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

References

Normative References

Informative References

Index

Property index

Issues Index