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websem

websem is the Web semantic compiler: it turns an SVG or HTML source into the resolved render contract, and it decides what this engine will and will not render. Everything upstream of it is parsing and cascading; everything downstream is painting. The judgement about meaning happens here.

source bytes
    -> one namespace-aware document      (csscascade)
    -> one Stylo cascade                 (csscascade)
    -> effective values: Base | Sample t (this crate)
    -> rframe::Frame                     (this crate)
    -> n0

Two entries, both retained. from_standalone_svg compiles an .svg document. from_html_inline_svg compiles an HTML document's first inline SVG — when that subtree is admitted the render succeeds and the rest of the page contributes nothing, which is a pinned contract rather than an accident. A retained session is compiled once and can then be read at Base or at an exact signed-nanosecond sample without re-parsing.

Module Ownership
svg the two entries, the element walk, the patrols, viewport mapping, shapes, paint
svg_path the d grammar, normalised to absolute commands
svg_transform the computed transform operation list, converted to one affine (the attribute grammar lives in csscascade)
svg_paint_server the gradient id table, href template chains, stops, and the fold of every gradient coordinate system into the contract's unit-box paints
svg_animation the closed exact-time sampling inventory
effective_values the Base-or-Sample view the compiler reads through

The admitted slice

Shapes <rect>, <circle>, <ellipse>, <path>, <line>, <polygon> and <polyline>, each with a solid fill and a solid stroke (width, cap, join, miter limit, and a path's fill rule — which the points shapes share). Containers <g> with the whole transform grammar in both spellings — the attribute enters the one cascade as a presentation hint of the CSS transform property — flattened into a per-node affine. <use>/<defs> same-document references, expanded into the one tree before the cascade (csscascade's svg_use), rendered as containers of their shadow content. <linearGradient>/<radialGradient> paint servers, resolved through a whole-document first-id-wins table into the contract's gradient paints — concentric radials only, stops from attributes, gradientTransform through the one cascade as the transform property's hint on gradient elements. Root sizing per SVG2 §8.2 with the full preserveAspectRatio grammar. One exact-time <animate attributeName="x"> on a top-level <rect>.

crates/n0_cli/README.md is the single statement of record for that slice and what it refuses; this table is the compiler's map, not a second copy of it.

Two admissions, one compiler

Strict refuses on the first construct outside the slice — the harness that names the edge. Best-effort, the product default, compiles what it admits and declares everything else as a named degradation: a subtree construct is skipped by name at a stable path, a blocked dynamic surface that leaves Base honest resolves every sample to Base, and a beyond-inventory animation element — active at document load, so its target's authored state never honestly renders — skips its target in every view, declared at the target's path. Where nothing degrades the two are frame-identical, and a law checks that over the whole corpus.

Neither mode ever guesses a pixel. That is the invariant the patrols exist for: a construct the compiler cannot honour must refuse loudly or be declared, and a patrol that over-refuses is preferred to one that lets a wrong pixel through.

Anti-goals

  • Not a layout engine. No box model, no flow, no intrinsic sizing. The HTML entry compiles an inline SVG subtree; it does not lay out the page.
  • Not a text shaper. Text refuses; shaping, fonts and resources are the kernel's and are not admitted through here.
  • Not a painter. It emits rframe::Frame and never touches a canvas.
  • Not a second cascade. Property resolution belongs to Stylo via csscascade; this crate reads computed values and authored text, and adds no matcher of its own.
  • Not a coverage claim. The slice above is a bounded enumeration with a stated edge, not a statement about SVG conformance.

Boundaries worth knowing

The cascaded surface is patrolled for an enumerated property set; anything beyond it is a named open boundary rather than a silent pass. A <style> sheet is not attributable to one element without selector matching, so sheet-borne findings are document-level: strict refuses the document, best-effort declares once against the sheet. Lengths in units with no basis in this build (vw, ex, and their family) refuse rather than resolve against the cascade's pinned device.

Why this crate exists and what it succeeded is recorded in docs/wg/consolidation/svg-engine-of-record.md.