Title: CSS Snapshot 2023
Shortname: css-2023
Level: none
Status: NOTE
Prepare for TR: yes
Date: 2023-12-07
Group: CSSWG
Work Status: revising
URL: https://drafts.csswg.org/css-2023/
TR: https://www.w3.org/TR/css-2023/
Editor: Tab Atkins Jr., Google, http://xanthir.com/, w3cid 42199
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Florian Rivoal, Invited Expert, https://florian.rivoal.net, w3cid 43241
Editor: Chris Lilley, W3C, https://svgees.us/, w3cid 1438
Abstract: This document collects together into one definition all the specs that
	together form the current state of Cascading Style Sheets (CSS) as of 2023.
	The primary audience is CSS implementers, not CSS authors, as this definition
	includes modules by specification stability, not Web browser adoption rate.
Status Text:
	
	This document represents the state of CSS as of 2023.
	
Boilerplate: index no

spec:selectors-4; type:selector; text::lang()
spec:selectors-4; type:dfn; text:document language

What is CSS?

Cascading Style Sheets (CSS)

CSS is a language for writing [=style sheets=], and is designed to describe the rendering of structured documents (such as HTML and XML) on a variety of media. CSS is used to describe the presentation of a [=source document=], and usually does not change the underlying semantics expressed by its [=document language=].

Style sheet

A set of rules that specify the presentation of a document. Style sheets are written by an [=Author=], and interpreted by a [=User Agent=], to present the document to the [=User=].

Source document

The document to which one or more style sheets apply. A source document’s structure and semantics are encoded using a [=document language=] (e.g., HTML, XHTML, or SVG).

Author

An author is a person who writes documents and associated style sheets. An authoring tool is a [=User Agent=] that generates style sheets.

User

A user is a person who interacts with a user agent to view, hear, or otherwise use the document.

User Agent (UA)

A user agent is any program that interprets a document and its associated [=style sheets=] on behalf of a [=user=]. A [=user agent=] may display a document, read it aloud, cause it to be printed, convert it to another format, etc. For the purposes of the CSS specifications, a [=User Agent=] is one that supports and interprets [=Cascading Style Sheets=] as defined in these specifications.

Background: The W3C Process and CSS

Working Draft (WD)

This is the design phase of a W3C spec. The WG iterates the spec in response to internal and external feedback. The first official Working Draft is designated the “First Public Working Draft” (FPWD). In the CSSWG, publishing FPWD indicates that the Working Group as a whole has agreed to work on the module, roughly as scoped out and proposed in the editor's draft. The transition to the next stage is sometimes called “Last Call Working Draft” (LCWD) phase. The CSSWG transitions Working Drafts once we have resolved all known issues, and can make no further progress without feedback from building tests and implementations. This “Last Call for Comments” sets a deadline for reporting any outstanding issues, and requires the WG to specially track and address incoming feedback. The comment-tracking document is the Disposition of Comments (DoC). It is submitted along with an updated draft for the Director's approval, to demonstrate wide review and acceptance.

Candidate Recommendation (CR)

This is the testing phase of a W3C spec. Notably, this phase is about using tests and implementations to test the specification: it is not about testing the implementations. This process often reveals more problems with the spec, and so a Candidate Recommendation will morph over time in response to implementation and testing feedback, though usually less so than during the design phase (WD). Demonstration of two correct, independent implementations of each feature is required to exit CR, so in this phase the WG builds a test suite and generates implementation reports. The transition to the next stage is “Proposed Recommendation” (PR). During this phase the W3C Advisory Committee must approve the transition to REC.

Recommendation (REC)

This is the completed state of a W3C spec and represents a maintenance phase. At this point the WG only maintains an errata document and occasionally publishes an updated edition that incorporates the errata back into the spec.

Classification of CSS Specifications

Cascading Style Sheets (CSS) — The Official Definition

CSS Level 2, latest revision (including errata) [[!CSS2]]

This defines the core of CSS, parts of which are overridden by later specifications. We recommend in particular reading Chapter 2, which introduces some of the basic concepts of CSS and its design principles.

CSS Syntax Level 3 [[!CSS-SYNTAX-3]]

Replaces CSS2§4.1, CSS2§4.2, CSS2§4.4, and CSS2§G, redefining how CSS is parsed.

CSS Style Attributes [[!CSS-STYLE-ATTR]]

Defines how CSS declarations can be embedded in markup attributes.

Media Queries Level 3 [[!CSS3-MEDIAQUERIES]]

Replaces CSS2§7.3 and expands on the syntax for media-specific styles.

CSS Conditional Rules Level 3 [[!CSS-CONDITIONAL-3]]

Extends and supersedes CSS2§7.2, updating the definition of ''@media'' rules to allow nesting and introducing the ''@supports'' rule for feature-support queries.

Selectors Level 3 [[!SELECTORS-3]]

Replaces CSS2§5 and CSS2§6.4.3, defining an extended range of selectors.

CSS Namespaces [[!CSS3-NAMESPACE]]

Introduces an ''@namespace'' rule to allow namespace-prefixed selectors.

CSS Cascading and Inheritance Level 4 [[!CSS-CASCADE-4]]

Extends and supersedes CSS2§1.4.3 and CSS2§6, as well as [[CSS-CASCADE-3]]. Describes how to collate style rules and assign values to all properties on all elements. By way of cascading and inheritance, values are propagated for all properties on all elements.

CSS Values and Units Level 3 [[!CSS-VALUES-3]]

Extends and supersedes CSS2§1.4.2.1, CSS2§4.3, and CSS2§A.2.1–3, defining CSS's property definition syntax and expanding its set of units.

CSS Custom Properties for Cascading Variables Module Level 1 [[!CSS-VARIABLES-1]]

Introduces cascading variables as a new primitive value type that is accepted by all CSS properties, and custom properties for defining them.

CSS Box Model Level 3 [[!CSS-BOX-3]]

Replaces CSS2§8.1, §8.2, §8.3 (but not §8.3.1), and §8.4.

CSS Color Level 4 [[!CSS-COLOR-4]]

Extends and supersedes CSS2§4.3.6, CSS2§14.1, and CSS2§18.2, also extends and supersedes [[CSS-COLOR-3]], introducing an extended range of color spaces beyond sRGB, extended color values, and CSS Object Model extensions for color. Also defines the 'opacity' property.

CSS Backgrounds and Borders Level 3 [[!CSS-BACKGROUNDS-3]]

Extends and supersedes CSS2§8.5 and CSS2§14.2, providing more control of backgrounds and borders, including layered background images, image borders, and drop shadows.

CSS Images Level 3 [[!CSS-IMAGES-3]]

Redefines and incorporates the external 2D image value type, introduces native 2D gradients, and adds additional controls for replaced element sizing and rendering.

CSS Fonts Level 3 [[!CSS-FONTS-3]]

Extends and supersedes CSS2§15 and provides more control over font choice and feature selection.

CSS Writing Modes Level 3 [[!CSS-WRITING-MODES-3]]

Defines CSS support for various international writing modes, such as left-to-right (e.g. Latin or Indic), right-to-left (e.g. Hebrew or Arabic), bidirectional (e.g. mixed Latin and Arabic) and vertical (e.g. Asian scripts). Replaces and extends CSS2§8.6 and §9.10.

CSS Multi-column Layout Level 1 [[!CSS-MULTICOL-1]]

Introduces multi-column flows to CSS layout.

CSS Flexible Box Module Level 1 [[!CSS-FLEXBOX-1]]

Introduces a flexible linear layout model for CSS.

CSS Basic User Interface Module Level 3 [[!CSS-UI-3]]

Extends and supersedes CSS2§18.1 and CSS2§18.4, defining 'cursor', 'outline', and several new CSS features that also enhance the user interface.

CSS Containment Module Level 1 [[!CSS-CONTAIN-1]]

Introduces the 'contain' property, which enforces the independent CSS processing of an element’s subtree in order to enable heavy optimizations by user agents when used well.

CSS Transforms Level 1 [[!CSS-TRANSFORMS-1]]

Introduces coordinate-based graphical transformations to CSS.

CSS Compositing and Blending Level 1 [[!COMPOSITING]]

Defines the compositing and blending of overlaid content and introduces features to control their modes.

CSS Easing Functions Level 1 [[!CSS-EASING-1]].

Describes a way for authors to define a transformation that controls the rate of change of some value. Applied to animations, such transformations can be used to produce animations that mimic physical phenomena such as momentum or to cause the animation to move in discrete steps producing robot-like movement.

CSS Counter Styles Level 3 [[!CSS-COUNTER-STYLES-3]]

Introduces the ''@counter-style'' rule, which allows authors to define their own custom counter styles for use with CSS list-marker and generated-content counters [[CSS-LISTS-3]]. It also predefines a set of common counter styles, including the ones present in CSS2 and CSS2.1.

Fairly Stable Modules with limited implementation experience

Media Queries Level 4 [[MEDIAQUERIES-4]]

Extends and supersedes [[CSS3-MEDIAQUERIES]], expanding the syntax, deprecating most media types, and introducing new media features.

CSS Display Module Level 3 [[CSS-DISPLAY-3]]

Replaces CSS2§9.1.2, §9.2.1 (but not §9.2.1.1), §9.2.2 (but not §9.2.2.1), §9.2.3, and §9.2.4 (and lays the foundations for replacing §9.7), defining how the CSS formatting box tree is generated from the document element tree and defining the 'display' property that controls it.

CSS Writing Modes Level 4 [[CSS-WRITING-MODES-4]]

Extends and supersedes [[CSS-WRITING-MODES-3]], adding more options for vertical writing.

CSS Fragmentation Module Level 3 [[CSS-BREAK-3]]

Describes the fragmentation model that partitions a flow into pages, columns, or regions and defines properties that control it. Extends and supersedes CSS2§13.3.

CSS Box Alignment Module Level 3 [[CSS-ALIGN-3]]

Introduces properties to control the alignment of boxes within their containers in the various CSS box layout models: block layout, table layout, flex layout, and grid layout.

CSS Shapes Module Level 1 [[CSS-SHAPES-1]]

Extends floats (CSS2§9.5) to effect non-rectangular wrapping shapes.

CSS Text Module Level 3 [[CSS-TEXT-3]]

Extends and supersedes CSS2§16 excepting §16.3, defining properties for text manipulation and specifying their processing model. It covers line breaking, justification and alignment, white space handling, and text transformation.

CSS Text Decoration Module Level 3 [[CSS-TEXT-DECOR-3]]

Extends and supersedes CSS2§16.3, providing more control over text decoration lines and adding the ability to specify text emphasis marks and text shadows.

CSS Masking Module Level 1 [[CSS-MASKING-1]]

Replaces CSS2§11.1.2 and introduces more powerful ways of clipping and masking content.

CSS Scroll Snap Module Level 1 [[CSS-SCROLL-SNAP-1]]

Contains features to control panning and scrolling behavior with “snap positions”.

CSS Speech Module Level 1 [[CSS-SPEECH-1]]

Replaces CSS2§A, overhauling the (non-normative) speech rendering chapter.

CSS Scrollbars Styling Module Level 1 [[CSS-SCROLLBARS-1]]

Defines properties to influence the visual styling of scrollbars, introducing controls for their color and width.

CSS View Transitions Module Level 1 [[CSS-VIEW-TRANSITIONS-1]]

Defines the View Transition API, along with associated properties and pseudo-elements, which allows developers to create animated visual transitions representing changes in the document state.

Modules with Rough Interoperability

CSS Transitions Level 1 [[CSS-TRANSITIONS-1]] and CSS Animations Level 1 [[CSS-ANIMATIONS-1]].

Introduces mechanisms for transitioning the computed values of CSS properties over time.

CSS Grid Layout Module Level 1 [[CSS-GRID-1]]

Introduces a two-dimensional grid-based layout system, optimized for user interface design. In the grid layout model, the children of a grid container can be positioned into arbitrary slots in a predefined flexible or fixed-size layout grid.

CSS Grid Layout Module Level 2 [[CSS-GRID-2]]

Extends and supersedes [[CSS-GRID-1]], introducing “subgrids” for managing nested markup in a shared grid framework.

CSS Will Change Level 1 [[CSS-WILL-CHANGE-1]]

Introduces a performance hint property called 'will-change'.

Filter Effects Module Level 1 [[FILTER-EFFECTS-1]]

Introduces filter effects as a way of processing an element’s rendering before it is displayed in the document.

CSS Font Loading Module Level 3 [[CSS-FONT-LOADING-3]]

Introduces events and interfaces used for dynamically loading font resources.

CSS Box Sizing Level 3 [[CSS-SIZING-3]]

Overlays and extends CSS§10., expanding the value set of the sizing properties, introducing more precise sizing terminology, and defining with more precision and detail various automatic sizing concepts only vaguely defined in CSS2.

CSS Transforms Level 2 [[CSS-TRANSFORMS-2]]

Builds upon [[CSS-TRANSFORMS-1]] to add new transform functions and properties for three-dimensional transforms, and convenience functions for simple transforms.

CSS Lists and Counters Module Level 3 [[CSS-LISTS-3]]

Contains CSS features related to list counters: styling them, positioning them, and manipulating their value.

CSS Logical Properties and Values Level 1 [[CSS-LOGICAL-1]]

Introduces logical properties and values that provide the author with the ability to control layout through logical, rather than physical, direction and dimension mappings. Also defines logical properties and values for the features defined in [[CSS2]]. These properties are writing-mode relative equivalents of their corresponding physical properties.

CSS Positioned Layout Module Level 3 [[CSS-POSITION-3]]

Contains defines coordinate-based positioning and offsetting schemes of CSS: [=relative positioning=], [=sticky positioning=], [=absolute positioning=], and [=fixed positioning=].

Resize Observer [[RESIZE-OBSERVER-1]]

This specification describes an API for observing changes to element’s principal box's size.

Web Animations [[WEB-ANIMATIONS-1]]

Defines a model for synchronization and timing of changes to the presentation of a Web page. Also defines an application programming interface for interacting with this model.

CSS Fonts Module Level 4 [[!CSS-FONTS-4]]

Extends and supersedes CSS Fonts 3 and provides more control over font choice and feature selection, including support for OpenType variations.

CSS Color Adjustment Module Level 1 [[CSS-COLOR-ADJUST-1]]

This module introduces a model and controls over automatic color adjustment by the user agent to handle user preferences and device output optimizations.

CSS Conditional Rules Module Level 4 [[CSS-CONDITIONAL-4]]

Extends CSS Conditional 3 to allow testing for supported selectors.

CSS Cascading and Inheritance Level 5 [[CSS-CASCADE-5]]

Extends CSS Cascade 4 to add cascade layers.

CSS Levels

CSS Level 1

The CSS Working Group considers the CSS1 specification to be obsolete. CSS Level 1 is defined as all the features defined in the CSS1 specification (properties, values, at-rules, etc), but using the syntax and definitions in the CSS2.1 specification. CSS Style Attributes defines its inclusion in element-specific style attributes.

CSS Level 2

Although the CSS2 specification is technically a W3C Recommendation, it passed into the Recommendation stage before the W3C had defined the Candidate Recommendation stage. Over time implementation experience and further review has brought to light many problems in the CSS2 specification, so instead of expanding an already unwieldy errata list, the CSS Working Group chose to define CSS Level 2 Revision 1 (CSS2.1). In case of any conflict between the two specs CSS2.1 contains the definitive definition. Once CSS2.1 became Candidate Recommendation—effectively though not officially the same level of stability as CSS2—obsoleted the CSS2 Recommendation. Features in CSS2 that were dropped from CSS2.1 should be considered to be at the Candidate Recommendation stage, but note that many of these have been or will be pulled into a CSS Level 3 working draft, in which case that specification will, once it reaches CR, obsolete the definitions in CSS2. The CSS2.1 specification defines CSS Level 2 and the CSS Style Attributes specification defines its inclusion in element-specific style attributes.

CSS Level 3

CSS Level 3 builds on CSS Level 2 module by module, using the CSS2.1 specification as its core. Each module adds functionality and/or replaces part of the CSS2.1 specification. The CSS Working Group intends that the new CSS modules will not contradict the CSS2.1 specification: only that they will add functionality and refine definitions. As each module is completed, it will be plugged in to the existing system of CSS2.1 plus previously-completed modules. From this level on modules are levelled independently: for example Selectors Level 4 may well be completed before CSS Line Module Level 3. Modules with no CSS Level 2 equivalent start at Level 1; modules that update features that existed in CSS Level 2 start at Level 3.

CSS Level 4 and beyond

There is no CSS Level 4. Independent modules can reach level 4 or beyond, but CSS the language no longer has levels. ("CSS Level 3" as a term is used only to differentiate it from the previous monolithic versions.)

CSS Profiles

Requirements for Responsible Implementation of CSS

Partial Implementations

Implementations of Unstable and Proprietary Features

Experimentation and Unstable Features

Proprietary and Non-standardized Features

Market Pressure and De Facto Standards

Note that the CSSWG must still be consulted to ensure coordination across vendors and to ensure coherency review by the CSS experts from each vendor. Note also that rough interoperability still usually means painful lack of interop in edge (or not-so-edge) cases, particularly because details have not been ironed out through the standards review process.

Vendor-prefixing Unstable Features

When exposing such a standards-track unstable feature to the Web in a production release, implementations should support both vendor-prefixed and unprefixed syntaxes for the feature. Once the feature has stabilized and the implementation is updated to match interoperable behavior, support for the vendor-prefixed syntax should be removed.

Preserving the Openness of CSS

Implementations of CR-level Features

Safe to Release pre-CR Exceptions

• The flow-relative equivalents of the sizing properties ('width', 'height', etc.), the border properties, the margin and padding properties. See explanation and specification.
• The ''width/min-content'' and ''width/max-content'' keywords of the sizing properties. See decision and specification.
• The ''conic-gradient()'' gradient notation. See decision.
• The aspect-ratio property. [[!CSS-SIZING-4]]
• The 'translate', 'rotate', and 'scale' properties. [[!CSS-TRANSFORMS-2]]
• The 'hyphenate-character' property. [[!CSS-TEXT-4]]
• The color-mix() function. [[!CSS-COLOR-5]]
• The <>, defined in [[!CSS-COLOR-4]] and used for interpolation of linear, radial and conic gradients. [[!CSS-IMAGES-4]]
• The [=relative color=] syntax, defined in [[!CSS-COLOR-5]]

• Everything in CSS Animations Level 1 and CSS Transitions Level 1.
• The '':dir()'', '':lang()'', and '':focus-within'' pseudo-classes from [[SELECTORS-4]].

Indices

Terms Index

Selector Index

At-Rule Index

Property Index

Values Index