style-cascade

Computes, for a document and the stylesheets of its page, which declaration wins on which element: origin, !important, cascade layers, the style attribute, specificity and source order, then inheritance and var(). From that it derives the ComputedStyle of a11y-dom for the rendering tier, including the effective background colour behind text. It does no layout and runs no scripts.

Status

0.1.0, not yet published. Used by a11y-rules for the selector match of its stylesheet rules, and by barrierlab-cli as the rendering tier of a browser-free audit. astro-post-audit is the planned second host for the cascade (casoon/astro-post-audit#115).

Compared with Chrome’s getComputedStyle on five real pages (casoon.dev, the auditmysite and barrierlab sites, Hacker News, a Wikipedia article; 1280 px light, dark and 390 px wide where the styles differ): text colour (about 2,800 elements), effective background (about 1,600 where determined), font size and weight agree on all but two elements (Chrome’s 13 px default for monospace, see Limits). Hacker News showed that presentational attributes (bgcolor, <font color>) must count; they do now. Not compared: the luminance samples over gradients, and pages with heavy use of container queries or :has(), which come out undetermined.

Structure

Public surface

Item	Purpose
Cascade::new(sheets, Environment)	builds the cascade for one document
Cascade::specified	the specified value of a property: Set, Unset or Unknown
Cascade::computed_style	the ComputedStyle for Rendering::computed_style
Cascade::effective_background	the colour behind the text, None if it cannot be known
Cascade::backdrop_luminances	luminance samples where a gradient lies behind the text
selector::Selector, Specificity, Mode	selector matching; Resting for the cascade, Possible for “could this rule apply?”
parse_color, over, luminance	CSS colours (including oklch(), lab(), color-mix() in srgb and oklab) and compositing
Environment	the assumed window: size, colour scheme, reduced motion; optional hover, pointer, contrast and forced-colors (Unknown when unset)

Dependencies

Downward: a11y-dom, stylesheet-parse, media-query-parse. Upward: a11y-rules, hosts.

Limits

What cannot be determined without layout stays undetermined, never guessed:

• Unknown propagates. A rule whose selector or @media condition is not understood (:has(), @container, resolution queries, hover/pointer queries without a value in Environment) makes every property it declares unknown on the elements it could match, unless a certain rule with higher weight wins anyway.
• Backgrounds. A background image makes the colour unknown. A gradient gives luminance samples (each stop and the midpoints), not every pixel. opacity, filter and blend modes anywhere up the chain, and position: absolute|fixed or non-blur backdrop-filter in front of the first opaque surface, make the background unknown.
• No layout. Overlap, sizes and what an absolutely positioned box really sits on are not known.
• Approximations. @supports conditions count as true unless negated; layers (named, dotted, nested and anonymous, Cascade 5 §6.4) rank by first appearance in document order; a layer first seen only under a condition with unknown outcome (@container, unknown @media) or with an invalid name has no certain rank, so its rules make the properties they declare Unknown; colours outside sRGB are clipped, not gamut-mapped; the page is assumed in the default light scheme unless Environment says otherwise.
• @import. An unresolved @import makes all answers Unknown (except an inline !important) until the host inlines the imported sheet; presentational hints rank below all author rules, layered or not.
• Monospace size. Chrome shrinks the default size to 13 px for font-family: monospace; the cascade does not, so <code> can come out 16 px where Chrome has 13 px.
• Presentational attributes are read for bgcolor, background, <font color>, <font size>, <body text> and <body link>; other legacy attributes are ignored.
• Shorthands are expanded only for background, font, border*, text-decoration and list-style; all is ignored.
• One document per cascade. Caches are keyed by node ID.