CSS values, the property table, media queries and @supports

Normative behaviour of the value layer of RecursiveAI.OneHtml.Css, written so another language can reproduce it. It sits on the tokens and component values of css-syntax.md. The algorithms are those of CSS Values and Units 4, CSS Color 4 (with the Color 5 relative-color and color-mix() parts), CSS Variables 1, Media Queries 5 and CSS Conditional 3 as of 20 October 2026; this file records what those texts leave open and what is not implemented.

Property table

• Source. ed/css.json of the @webref/css@latest branch of <https://github.com/w3c/webref>, commit bb927df3d6cc86f1c0beca204844a170a0843a18 (committed 2026-10-22), SHA-256 30cb8daa786d6c27a1f74c5d413311c785ee0eec1dd34c65e8a4e6681c86baca. This is webref's merged extract of every CSS specification's property definitions, not the per-spec curated files.
• Generator. tools/generate-css-properties.py <css.json> writes src/OneHtml/Css/Generated/PropertyTableData.g.cs. It checks the input against the pinned SHA-256, is deterministic, and records commit and date in the generated file. The fetch commands are in the script's header. tools/generate-css-colors.py does the same for the named-color table of CSS Color 4 (Generated/NamedColors.g.cs).
• Content. 822 properties, of which 170 are shorthands, 64 are logical-property members (20 logical groups), and 64 are legacy aliases. Each entry has its name, inheritance (yes, no, "see longhands", unspecified), initial value as text, applies-to, percentages, computed-value text, value syntax, alias target, longhands, reset-only longhands and logical group. The initial value is parsed to component values on first use.
• Prose values. An initial value is "text" only when the source gives a value that is CSS. Otherwise the entry says which kind of prose it is and the initial value is not guessed:
  • see longhands (98): all, animation, background, border and its sides, color-adjust, columns, container, font, gap, list-style, margin-block, margin-inline, mask, offset, outline, padding-block, padding-inline, text-decoration, transition, the rule* and column-rule* families, the corner-*-shape family, the timeline-trigger* and *-timeline shorthands, place-items, cue, pause, rest, and the others the generator reports with PropertyInitialKind.SeeLonghands;
  • depends on the user agent (1): font-family;
  • not defined for shorthand properties (1): marker;
  • missing in the source (73): the -webkit- aliases of the flexbox, animation, transition, transform, mask, box and appearance properties, and column-width, font-stretch, grid-column-gap, grid-gap, grid-row-gap, stop-color, stop-opacity, word-wrap. PropertyTable.All.Where(p => p.InitialKind != PropertyInitialKind.Value) lists them.
• Typed properties. 33 properties have a typed parser (TypedProperties.IsTyped): the accessibility set (display, visibility, content, color and the other color properties, background-image, opacity, font-size, font-weight, font-style, font-family, outline-*, clip, clip-path, position and the offsets, overflow-*, speak, speak-as, pointer-events). Every other property keeps its value as component values.
• Licences. See THIRD-PARTY-NOTICES.md.

CSS-wide keywords and shorthands

• initial, inherit, unset, revert and revert-layer are recognised only when they are the whole value (CssWideKeywords.Detect).
• Shorthands.Expand(name, value) returns the leaf longhands in table order; nested shorthands are flattened and reset-only longhands are initial. Status is Expanded, Invalid (the value does not match the grammar), PendingSubstitution (the value contains var() and cannot be split yet), NotExpanded (a shorthand without a grammar here: keep the value as written) or NotShorthand.
• Grammars exist for: shorthands whose value syntax ends in {1,4} or {1,2} with that many longhands (margin, padding, inset, scroll-margin, border-color, border-style, border-width and the logical pairs) and the two-longhand pair shorthands; shorthands with exactly a -width, -style and -color longhand (border and its sides, outline, column-rule); and text-decoration, list-style, font, background, background-position, transition and animation. Shorthands.HasGrammar(name) says which. Every other shorthand is NotExpanded.
• Where the table's syntax ends in {1,4} or {1,2} the expansion uses the CSS rule for omitted sides.

Numbers, units and math

• A CssQuantity is a number, percentage or dimension as written, or the result of a math function. A literal too large for a double is held at the largest finite value; only a math function yields infinity.
• Units: absolute lengths, font-relative (em ex cap ch ic lh rem rex rcap rch ric rlh), viewport (`vw vh vi vb vmin vmaxand thes,l,dvariants), container (cqw cqh cqi cqb cqmin cqmax`), angle, time, frequency, resolution and flex units. LengthContext supplies the inputs for relative units; the library never measures fonts. When font metrics are unknown, ex=0.5em, ch=0.5em, cap=0.7em and ic=1em (Values 4 §6.1.1 fallbacks; cap is this library's choice). Absolute units use the CSS reference pixel.
• Math functions: calc min max clamp round mod rem sin cos tan asin acos atan atan2 pow sqrt hypot log exp abs sign. Parsing uses an explicit stack, so nesting never recurses; the expression is capped at 65,536 nodes.
  • Type checking follows Values 4 §10.1: sums need the same type; a percentage in a calculation is invalid where the property takes no percentage, even when it cancels (sign(50%)).
  • Simplification: constants fold; min/max/clamp of constants with the same unit fold, percentages included; a sum is sorted number, percentage, then dimensions by unit name (§10.13). A clamp() bound written none stands alone and takes no part in arithmetic.
  • Serialization: a math function that folds to a constant serializes as calc(<value><unit>); NaN and infinity as calc(NaN), calc(infinity).
  • Not implemented: products of two dimensions (1px * 1px) are kept as written rather than simplified, progress(), random(), random-item(), calc-mix(), inherit() and attr() in math (Values 5).

Colors

• Forms: named colors, hex (3, 4, 6, 8 digits), rgb() rgba() hsl() hsla() hwb() lab() lch() oklab() oklch(), color() with the predefined spaces except display-p3-linear, color-mix(), relative color syntax, light-dark(), currentcolor, transparent and the system colors.
• Legacy serialization (hex, named, rgb(), hsl(), hwb(), with no none): rgb(r, g, b) or rgba(r, g, b, a) with channels rounded half up to integers 0..255 and the alpha stored as 8 bits and written with the two-decimal rule of Color 4 §16.1.1 (#00000088 is rgba(0, 0, 0, 0.53)).
  • css-parsing-tests expects unrounded channels and a six-decimal alpha for these (it predates the 8-bit rule). The suite runner accepts a difference within the rounding; the current text wins.
• Missing components. rgb() with none serializes as color(srgb ...) with none kept; hsl() and hwb() stay in their own function with none kept (§16.2.2).
• Modern forms print their own function with numbers rounded to six decimals, shortest form. Color 4 requires at least round-trip precision, so six decimals is allowed; tests that expect six significant digits (73.3386) differ. Numbers of magnitude 1e15 or more use an exponent.
• Parse-time clamps: lab/lch lightness to 0..100 and oklab/oklch to 0..1, chroma at least 0, hue taken modulo 360, hsl saturation and lightness to 0..100, hwb whiteness and blackness at least 0 (a sum of 100% or more is a gray), NaN from math becomes 0, an infinite hue becomes 0. rgb() channels clamp to 0..255.
• color-mix() in hsl or hwb returns sRGB. Hues interpolate with the four methods. Missing components take the other operand's value.
• Relative colors: channel keywords are numbers (rgb 0..255, hsl and hwb percentages as numbers 0..100). The result of rgb(), hsl() or hwb() relative syntax is color(srgb ...) unless a component is missing, when hsl and hwb keep their function. A keyword used alone keeps its component missing.
• Powerless hues. Converting into hsl, hwb, lch or oklch marks the hue missing when the source chroma is at most 0.0015 (lab, lch) or 0.000004 (oklab, oklch), the converted saturation is under 0.001%, or whiteness plus blackness is within 0.002% of 100%. An authored color already in the mix space keeps its hue.
• System colors take fixed light-scheme sRGB values (dark with ColorParseOptions.DarkScheme) and keep their keyword in SystemName; ToCss() prints the keyword.
• Gamut mapping to sRGB (ToSrgb) is the CSS Gamut Mapping algorithm in OKLCH with the JND of 0.02.
• Not implemented: alpha(), contrast-color(), color-layers(), display-p3-linear, single-operand color-mix(), color-mix() with 0% weights, and the specified-value serialization of color-mix(), relative colors and math functions inside colors (ToCss() returns the resolved color; lab(calc(0.5) ...) prints the number). A font-relative or container length inside a color calculation does not parse, because the color parser has no length context.

var()

• VarSubstitution.ResolveCustomProperties computes custom properties (cycles and references to invalid properties become guaranteed-invalid and are left out). Substitute replaces var() in a value. Both are iterative.
• A cycle is a strongly connected component (Tarjan, iterative) of the graph whose edges are the var() references in a value, fallbacks included, whether or not the fallback would be used.
• A fallback has its leading whitespace dropped. A substituted value gets /**/ between two pieces only where two name-like characters would fuse into one token.
• Limits, checked before the work they guard (VarLimits): 1,048,576 characters in one substituted value, 2,097,152 token visits (every token of every expanded fallback counts, so an empty expansion is bounded too), and 16,777,216 characters over one ResolveCustomProperties call. Exceeding one gives SubstitutionStatus.LimitExceeded, and the declaration is invalid at computed-value time. A billion-laughs input fails in milliseconds.
• IsValidSyntax checks var() at parse time (Variables 2): the first argument is one or more tokens, or a single non-empty {} block; a {} block cannot be mixed with other tokens there; whatever follows the first top-level comma is the fallback and may be empty. Whether the first argument names a custom property is checked at substitution, which accepts only a --name identifier (Variables 1).

Media queries

• MediaQueryList.Parse follows Media Queries 5: media types, not/only/and/or, range syntax (width >= 400px, 400px < width <= 800px), <general-enclosed> (unknown). ToCss() is the canonical text; an invalid query serializes and evaluates as not all.
• Evaluation is three-valued. A query whose condition is unknown does not match, and not of unknown is unknown, so not all and (unknown-feature) does not match.
• Unclosed blocks, functions and strings are closed at the end of the input, and the canonical text writes those closers out so that it parses back to itself.
• Features: `width height device-width device-height aspect-ratio device-aspect-ratio resolution color color-index monochrome grid (0 or 1) orientation hover any-hover pointer any-pointer update overflow-block overflow-inline scan color-gamut prefers-color-scheme prefers-reduced-motion prefers-reduced-transparency prefers-reduced-data prefers-contrast forced-colors scripting display-mode dynamic-range video-dynamic-range inverted-colors nav-controls` and -webkit-device-pixel-ratio. Other names, and values outside a feature's grammar, are unknown.
• @container queries are parsed as at-rules and are not evaluated (they need layout).

StyleContext

StyleContext is an immutable record. Today it holds MediaFeatures: media type, width, height, device pixel ratio, prefers-color-scheme, prefers-reduced-motion, -transparency, -data, prefers-contrast, forced-colors, hover/any-hover, pointer/any-pointer, color bits, and scripting, which is always none because the library never runs script. The defaults are a 1280x720 light-scheme screen. New members get defaults, so existing callers keep working when the interaction state (focus, hover, :target, :active) is added.

@supports

SupportsCondition parses not, and, or, parentheses, (property: value), selector(...), and <general-enclosed> (false). DeclarationSupport.IsSupported(property, value) is true when:

1. the property is a custom property with a name; or
2. it is in the property table (aliases included) and the value is non-empty and syntactically acceptable (no bad strings or URLs, no stray closers, no top-level braces or semicolons), and
   • the value is a CSS-wide keyword or contains var(); or
   • the property is typed: its typed parser accepts the value; or
   • the property is a shorthand with a grammar: it expands; or
   • the property's value syntax is only keywords and simple types (<length>, <color>, ...): the value matches it; or
   • otherwise: nothing more is checked (the value is accepted).

selector(...) is decided by SupportsOptions.SelectorSupported, which gets the selector text; without a callback the text only has to look like one complex selector (selector-shaped tokens, no top-level commas). The selectors component is meant to supply the real check.

Linked stylesheets

WhatwgCssEncodingProvider.Instance implements the CSS Syntax §3.2 byte-to-text steps with the WHATWG decoders: the protocol label, the @charset rule, then the referrer's encoding and UTF-8. The decoder replaces CR and CRLF with LF.

Deviations the test suites show

The replay of the web-platform-tests files (testdata/css/wpt-expectations.txt) and of css-parsing-tests lists every known failure with its reason. In short: the specified value of color-mix(), relative colors and math in colors is not kept; Values 5 and Color 6 functions are missing; typed arithmetic on dimensions is not simplified; the analogous- component carry-forward of Color 4 §12.4 between different spaces is incomplete.