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csscascade

csscascade is the engine's internal Stylo bridge. It parses a namespace-aware document, adapts that document to Stylo's DOM traits, and resolves browser-grade computed CSS values. It does not perform layout or painting.

The crate has one implementation:

Module Ownership
dom Frozen, arena-allocated DemoDom parsed by html5ever
adapter Stylo and selectors trait implementations over that DOM
cascade Stylesheet collection, Stylo setup, and computed-style traversal

DemoDom and the Html* adapter names predate their use for namespace-aware HTML and SVG content. They are current implementation names, not a claim that the production surface is HTML-only.

Pipeline

source bytes
    -> DemoDom
    -> DocumentSession
    -> CascadeDriver::new(&mut session).style_document()
    -> computed values attached to element data
    -> session-bound read handles
    -> semantic consumer

The semantic consumer owns normalization into its downstream contract. csscascade never owns a renderer IR, layout tree, graphics backend, resource loader, or source I/O policy.

DocumentSession owns one frozen DOM and its Stylo lock/data. HtmlDocument, HtmlNode, and HtmlElement are pointer-sized Copy handles whose Rust lifetime is borrowed from that session. Their node identity includes the session, so arena-local identifiers from separate live documents cannot cross-resolve. Styling exclusively borrows the session and consumes the CascadeDriver; read handles can exist before or after that pass, never alongside mutation-capable cascade state.

Diagnostic

The retained example exercises the live path and prints resolved longhands:

cargo run -p csscascade --example resolve_and_print
cargo run -p csscascade --example resolve_and_print -- fixtures/test-html/L0/hello.html

Known constraints

  • Cascade environment values such as viewport, device-pixel ratio, color scheme, and pointer capabilities are not yet a complete explicit host input.
  • External stylesheets and other resources are not loaded here. A host must declare and resolve resources outside the pure cascade operation.
  • SVG paint properties are available through the workspace's official Stylo revision, but source-to-cascade ingress and semantic consumption remain producer work.
  • The session is deliberately not declared Send or Sync: Stylo's computed element data uses interior mutation during the exclusive cascade pass.

The consolidation constraints are defined by the Web-First Amendment. The current module and lifetime details are recorded in ARCHITECTURE.md.

License

MIT or Apache-2.0