Stable CSS Modules as of September 2021 (via ChatGPT4, so take with a grain of salt):
- CSS Selectors Level 3 - Describes patterns to select and style elements in the document tree.
- CSS Selectors Level 4 (some features) - Enhancements to selectors, including matching more complex patterns and relationships.
- CSS Syntax Level 3 - Defines the syntax of CSS and rules for parsing it.
- CSS Cascade and Inheritance Level 3 & 4 - Explains how to compute the value of a property on an element or pseudo-element.
- CSS Color Level 3 & 4 - Defines color properties, values, and transparency.
- CSS Namespaces - Describes how to use XML namespaces in CSS.
- CSS Backgrounds and Borders Level 3 - Describes how to style backgrounds and borders of elements.
- CSS Values and Units Level 3 - Describes numeric, percentage, and position data types and their functions.
- CSS Display Level 3 - Describes the 'display' property, which determines the type of rendering box for an element.
- CSS Box Model Level 3 - Describes the rectangular boxes that layout the web's visual interface.
- CSS Text Level 3 - Deals with text manipulation and how text interacts with the elements that contain it.
- CSS Fonts Level 3 - Describes font properties and dynamic font loading.
- CSS Writing Modes Level 3 & 4 - Describes international writing modes like left-to-right, right-to-left, and vertical.
- CSS Flexible Box Layout Level 1 - Describes the flex layout model for complex layouts and interface components.
- CSS Grid Layout Level 1 - A two-dimensional grid-based layout system, optimized for user interface design.
- CSS Multi-column Layout - Allows content to flow into multiple columns with a gap and optional rule between them.
- CSS Positioned Layout Level 3 - Describes the positioning schemes.
- CSS Transitions - Provides a way to animate changes in CSS properties.
- CSS Animations - Allows the animation of CSS properties over time.
- CSS 2D Transforms - Describes how to transform elements in a two-dimensional space.
- CSS 3D Transforms - Describes how to transform elements in a three-dimensional space.
- CSS Masking Level 1 - Describes how to partially or fully hide portions of visual elements.
- CSS Compositing and Blending Level 1 - Describes how elements should blend into their backdrop.
- CSS Shapes Level 1 - Describes geometric shapes for use in advanced layout and positioning.
- CSS Pseudo-elements Level 4 - Describes the syntax for pseudo-elements, which represent elements not present in the document tree.
- CSS Image Values and Replaced Content Level 3 - Deals with gradients, image generation, and replaced content manipulation.
- CSS Images Level 3 - Defines syntax and semantics of the properties and values related to images.
- CSS Containment Level 1 - Indicates an element's subtree is independent of the rest of the layout.
- CSS Basic User Interface Level 3 (CSS3 UI) - Describes user interface related properties and values.
- CSS Speech - Describes how documents (especially XHTML) are to be presented in speech.
- CSS Page Floats - Discusses pagination and layout of content in paged media.
- CSS Ruby Layout Level 1 - Describes the layout model for ruby annotations used in East Asian typography.
- CSS Overflow Level 3 - Describes the 'overflow' and 'clip' properties.
- CSS Conditional Rules Level 3 - Describes conditions for applying part of a stylesheet based on capabilities of the processor.
- CSS Lists and Counters Level 3 - Describes list styling and counter properties.
- CSS Counter Styles Level 3 - Introduces '@counter-style', a rule for defining counter styles.
- CSS Will Change - Gives a hint to the browser about what aspects of an element will change.
- CSS Line Grid - Describes a grid-based system for aligning lines of text.
- CSS Logical Properties and Values Level 1 - Introduces a model to support international writing modes.
- CSS Box Alignment Level 3 - Describes properties relating to the alignment of boxes in their containers.
- CSS Box Sizing Level 3 - Describes the sizing properties of boxes, including 'box-sizing'.
This list encompasses many of the stable CSS modules as of 2021.
As of 2021, several CSS modules were still in the working draft stage or were proposals.
- CSS Scrollbars Level 1 - Introduces properties to style scrollbars.
- CSS Grid Layout Level 2 - Extends the grid layout model with subgrids and other features.
- CSS Spatial Navigation Level 1 - Deals with navigating between focusable elements using arrow keys.
- CSS Text Level 4 - Further developments in text manipulation and interaction with its containing elements.
- CSS Box Model Level 4 - Proposes new features related to the box model, including margin and padding properties.
- CSS Cascading and Inheritance Level 5 - Further enhancements to how styles are cascaded and inherited.
- CSS Color Level 5 - Proposes new color functions and extensions to color values.
- CSS Conditional Rules Level 4 - New concepts regarding conditional application of CSS, including container queries.
- CSS Fonts Level 4 - Introduces new features for font loading and more font properties.
- CSS Images Level 4 - Extensions to image-related properties and new image functions.
- CSS Overflow Level 4 - New concepts around how overflowed content behaves.
- CSS Pseudo-elements Level 5 - Introduces new pseudo-elements and expands on existing ones.
- CSS Scroll Snap Level 1 - Describes how to control the behavior of scroll containers.
- CSS Sizing Level 4 - Introduces new properties and values for sizing elements.
- CSS Display Level 4 - Expands on the 'display' property with more values and concepts.
- CSS Tables Level 3 - Refines the styling and layout of tables.
- CSS Values and Units Level 4 - Introduces new units and functions for values.
- CSS Box Alignment Level 4 - Expands on the alignment of boxes in their containers.
- CSS Shadow Parts - Allows styling of shadow parts in a web component.
- CSS Containment Level 2 - Extends the containment features to ensure an element's layout does not affect other elements.
- CSS Custom Highlight API Level 1 - Proposes a way to style specific text ranges.
- CSS Environment Variables Level 1 - Introduces the concept of environment variables in CSS.
- CSS Fill and Stroke - Proposes properties to define how SVG contents are filled and stroked.
- CSS Inline Layout Level 3 - Details on how inline boxes are laid out in the formatting context.
- CSS Nesting Module - Proposes a native mechanism to nest one style rule inside another.
- CSS Pseudo-classes Level 4 - Introduces new pseudo-classes and further definitions on existing ones.