CSS Specification Conventions

Unofficial Proposal Draft,

This version:
https://xfq.github.io/specs/css-conventions/
Issue Tracking:
Inline In Spec
GitHub Issues
Editor:
Fuqiao Xue

Abstract

This document defines the notational conventions that are used in CSS specifications.

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

1. Introduction

TBD

2. Property definition conventions

Each CSS property definition begins with a summary of key information that resembles the following:

Name: property-name
Value: legal values & syntax
Initial: initial value
Applies to: elements this property applies to
Inherited: whether the property is inherited
Percentages: how percentage values are interpreted
Media: which media groups the property applies to
Computed value: how to compute the computed value
Canonical order: the order of serializing components of shorthand properties
Animation type: TBD

2.1. Value

This part specifies the set of valid values for the property whose name is property-name. See [CSS3VAL] for information about the value definition syntax.

TODO: link to css-syntax-3?

2.2. Initial

This part specifies the property’s initial value. [CSS-CASCADE-4]

2.3. Applies to

This part lists the elements to which the property applies. All elements are considered to have all properties, but some properties have no rendering effect on some types of elements. For example, the clear property only affects block-level elements.

2.4. Inherited

This part indicates whether the value of the property is inherited from an ancestor element. [CSS-CASCADE-4]

2.5. Percentages

This part indicates how percentages should be interpreted, if they occur in the value of the property. If "N/A" appears here, it means that the property does not accept percentages in its values.

2.6. Media

This part indicates the media groups to which the property applies. Information about media groups is non-normative.

2.7. Computed value

This part describes the computed value for the property. [CSS-CASCADE-4]

2.8. Canonical order

TBD

2.9. Animation type

"Animatable" vs "Animation type"? See https://github.com/w3c/csswg-drafts/issues/72

3. Descriptor definition conventions

Abstract out the notion of descriptor from font descriptors, viewport descriptors, media features etc.

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:

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.

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

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level 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 public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-CASCADE-4]
Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. URL: https://www.w3.org/TR/css-cascade-4/
[CSS22]
Bert Bos. Cascading Style Sheets Level 2 Revision 2 (CSS 2.2) Specification. URL: https://www.w3.org/TR/CSS22/
[CSS3VAL]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. URL: https://www.w3.org/TR/css-values-3/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

Property Index

Name Value Initial Applies to Inh. %ages Media Anim­ation type Canonical order Com­puted value
property-name legal values & syntax initial value elements this property applies to whether the property is inherited how percentage values are interpreted which media groups the property applies to TBD the order of serializing components of shorthand properties how to compute the computed value

Issues Index

TODO: link to css-syntax-3?
"Animatable" vs "Animation type"? See https://github.com/w3c/csswg-drafts/issues/72
Abstract out the notion of descriptor from font descriptors, viewport descriptors, media features etc.