This module contains CSS features for aligning content size to multiple of unit size.
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
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.
This document is a First Public Working Draft.
Publication as a First Public Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-rhythm” in the title, preferably like this: “[css-rhythm] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.
This specification provides features to control sizes of CSS objects according to the rules desired by use cases.
Controlling sizes of CSS objects to be multiple of a unit is desired in many cases. This level of the specification focuses on following cases.
Adjust heights of line boxes to multiples of the specified length.
Adjust heights of block-level boxes to multiples of the specified length.
By controlling heights of line boxes, lines of text in different fonts can create consistent visuals to help readability.
Also by stacking such line boxes, authors can align lines across columns, pages, scroll-snapped blocks, or multiple blocks placed absolutely, to produce vertical rhythm.
Vertical rhythm kept through pictures and different size of text in a multi-column document.
This property defines the step unit for line box heights. When the step unit is set to a positive <length>, the line box heights are rounded up to the closest multiple of the unit. Negative <length> values are invalid.
[CSS21]§10.8 Line height calculations defines how to compute the height of a line box from its inline-level content. The rounding is applied to the resulting height of the line box, and the additional space is distributed to over-side and under-side of the line box equally, so that the original line box appears at the center of the multiple of step unit. This adjustment is done by assuming that there is an inline-level box that has adjusted A' and D' in the line box. This inline-level box does not affect alignment points of the vertical-align property, except values that align relative to the line box.
Rounding up the computed line box height.
none and 0 are equivalent. Is this ok?
Should this be animatable? There doesn’t seem to be use cases but needed for consistency?
In the following example, the height of line box in each paragraph is rounded up to the step unit.
It is usually recommended to set the line-height lower than the step unit. The used line height can increase due to several factors such as the use of vertical-align or font fallback.
3. Adjusting Block-level Box Heights
This proposal can be simplified down to just the block-step-size property, represented solely through its shortened form as block-step. This level will likely at most contain block-step-size and block-step-insert, leaving block-step-align and block-step-round to be added if the future demands. The full design is described herein for current discussion and future reference.
This proposal is currently defined to apply only to block-level boxes. This limitation is solely to simplify the first iteration; it should eventually be extended to all layout modes that honor specified heights.
This property defines the step unit for a block-level box’s block size. When the step unit is set to a positive <length>, the box’s outer height is rounded (see block-step-round) to the closest multiple of the unit. Negative <length> values are invalid.
Values other than none cause the box to establish a new formatting context.
In situations where margins collapse, only the box’s own margin is considered in calculating its outer size.
This property specifies whether extra spacing derived from applying block-step-size is inserted inside (like padding) or outside (like margin) the box’s border.
Values have the following meanings:
margin
Any extra space resulting from a block-step-size-induced adjustment is inserted outside the box’s border, as extra margin.
padding
Any extra space resulting from a block-step-size-induced adjustment is inserted inside the box’s border, as extra padding.
This property specifies whether extra spacing derived from applying block-step-size is inserted before, inserted after, or split between both sides of the box.
This property specifies whether adjustments due to block-step-size insert positive or negative space.
Values have the following meanings:
up
The outer size of the box is increased (positive space is inserted) to fulfill the block-step-size constraint.
down
The outer size of the box is decreased (negative space is inserted) to fulfill the block-step-size constraint.
nearest
The outer size of the box is either increased (as for up) or decreased (as for down—whichever results in the smallest absolute change—to fulfill the block-step-size constraint. If both options would result in the same amount of change, the size is increased.
3.5. Block Step Adjustment Shorthand: the block-step shorthand
Authors are advised to use this shorthand rather than the longhands unless there is a specific need for its individual longhands to cascade independently.
4. Privacy and Security Considerations
This specification introduces no new privacy leaks, or security considerations beyond "implement it correctly".
5. Acknowledgments
This specification would not have been possible without the help from: Takao Baba, Chris Eppstein, Shinyu Murakami, Tsutomu Nanjo, Charlie Neely, Florian Rivoal, Hiroshi Sakakibara, Alan Stearns, and the CSS Working Group members.
Conformance
Document conventions
Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.
All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.
Conformance classes
Conformance to this specification is defined for three conformance classes:
A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.
A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.
Requirements for Responsible Implementation of CSS
The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.
Partial Implementations
So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.
Implementations of Unstable and Proprietary Features
Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.
To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.
Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the [email protected] mailing list.
Should this be animatable? There doesn’t seem to be use cases but needed for consistency? ↵
This proposal can be simplified down to just the block-step-size property, represented solely through its shortened form as block-step. This level will likely at most contain block-step-size and block-step-insert, leaving block-step-align and block-step-round to be added if the future demands. The full design is described herein for current discussion and future reference. ↵
This proposal is currently defined to apply only to block-level boxes. This limitation is solely to simplify the first iteration; it should eventually be extended to all layout modes that honor specified heights. ↵
