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 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.
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 specifies whether adjustments due to block-step-size insert positive or negative space.
Values have the following meanings:
The outer size of the box is increased (positive space is inserted)
to fulfill the block-step-size constraint.
The outer size of the box is decreased (negative space is inserted)
to fulfill the block-step-size constraint.
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".
This specification would not have been possible without the help from:
and the CSS Working Group members.
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",
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 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.
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
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. ↵