Finding: SVG paint in the shared cascade

Genre: finding and decision evidence. Not a spec and not a plan. It records what was established while building the Web-first prototype, the upstream change that followed, and the scope of the resulting owner decision.

Status: D-L taken 2026-07-23 in the charter's registry. SVG paint uses Servo-capable support maintained in official upstream Stylo. A published release containing the required support is preferred; until one exists, the dependency is fixed to an immutable official-upstream revision. A floating branch and a private source fork are outside the decision.

The crux

The Web-first direction requires HTML and SVG to share one browser-grade cascade, so that a rule like .mark { fill: … } authored anywhere in the document reaches an SVG descendant through the same cascade that styles HTML. The prototype proves the cascade crosses the boundary — an HTML <style> rule reaches an inline-SVG element — but only for properties the cascade actually models. The released Stylo version used by that prototype did not model SVG paint under Servo, so its SVG semantic compiler read only direct fill presentation attributes outside the cascade and dropped an inline style declaration for fill as an unknown longhand. Official upstream has since enabled the basic paint set under Servo. That resolves dependency provenance; it does not by itself supply production SVG/XML ingress, consume the computed values, or land an SVG capability.

Evidence

• E1 — at the prototype baseline, 44 of the 46 longhands in Stylo's SVG style structs were absent under the compiled engine. Stylo splits its property database by engine (servo vs gecko). The workspace compiles the servo engine; property declarations marked gecko-only are never registered, so they do not exist in the computed style. Of the 46 SVG-struct longhands in Stylo 0.16, 44 are gecko-only — the whole of fill, fill-opacity, fill-rule, stroke, stroke-width, stroke-dasharray, stroke-linecap/-linejoin/-miterlimit/-opacity/ -dashoffset, paint-order, marker-start/-mid/-end, text-anchor, clip-rule, color-interpolation(-filters), shape-rendering, vector-effect, stop-color/-opacity, flood-/lighting-color, the SVG geometry presentation properties (x,y,cx,cy,r,rx,ry,d), and the SVG mask longhands. Only clip-path and mask-image — shared box properties, not SVG paint — survive under servo. A fill declaration in a stylesheet or inline style is therefore an unknown declaration, dropped at parse time; there is no computed value to read.

• E2 — CSS custom properties do cascade and read back under servo, but as untyped token strings. A custom property set on an HTML <style> rule (.mark { --x: #16a34a }) cascades to an inline-SVG descendant with full inheritance and specificity and is readable from the computed custom-property map — verified empirically. But the value returns as the raw token string ("#16a34a"): no computed-value processing, no currentColor resolution against color, no url(#…) paint-server binding, no type checking, and custom properties inherit by default with their own invalidation semantics. The carrier reproduces cascade mechanics for a paint value; it does not reproduce SVG paint computed values.

• E3 — the gecko engine does not satisfy this program's build constraint. Enabling Stylo's gecko engine (which registers all the SVG longhands) pulls bindgen, mozbuild, and nsstring — the Mozilla/Gecko build system and its C++ bindings. That is not a standalone Cargo/servo build; adopting it would import Gecko's build environment into the engine.

• E4 — a minimal servo-only fork is build- and behavior-feasible for fill and stroke. A bounded Stylo 0.16 spike enabled only these two inherited longhands under servo and classified changes to them as repaint damage. The generated layout guard exposed one additional pointer-sized computed-style slot: after recording the expected 224-to-232-byte change, a servo-only build passed without bindgen, mozbuild, or nsstring. The cascade then produced native typed SVG-paint representations for colors, none, and a URL with a color fallback; the initial values remained black and none. It also proved inheritance, syntactically-invalid fallback to a lower declaration, and the distinct invalid-at-computed-value behavior of an unresolved var() (unset, then inherited, without reconsidering the lower declaration). currentColor remained typed through computed style; invoking Stylo's color-resolution operation on the consuming child's style resolved it against that child's computed color. The paint-server case proves typed URL and fallback preservation, not resource binding. This evidence covers two longhands, not the other gecko-only SVG longhands or their servo glue.

• E5 — the three independent intake gaps are technically closeable without a second cascade. Under HTML foreign-content parsing, a bounded spike admitted SVG-namespace style elements in document order and synthesized an empty-namespace fill presentation attribute as exactly one declaration at presentation-hint precedence. Author normal over presentation, inline normal over presentation, author important over inline normal, and later equal- specificity SVG stylesheet order all passed. A semicolon-bearing attribute was rejected as one invalid paint value rather than expanded into additional declarations. A separate strict SVG/XML spike populated the same frozen semantic DOM and used the same Stylo cascade. It proved a direct SVG document root with no synthetic HTML, non-HTML document mode, :root, SVG and XLink namespaces, case-sensitive element and attribute selectors, SVG-namespace stylesheet intake, inheritance, rejection of malformed XML, and rejection of wrong-case or wrong-namespace SVG roots. The selected spike parser accepts UTF-8 input and rejects DTDs; a production entry still requires deliberate non-UTF-8 and DTD policies, plus base-URL and external-resource policies. These are isolated feasibility results, not capabilities landed in the workspace.

• E6 — the current direct-attribute fallback is narrower than previously stated. It reads a fill attribute and resolves currentColor through the computed color. It does not parse fill from an inline style, so only the direct-attribute case is in hand. This distinction matters because the status quo does not cover both authoring forms.

• E7 — official upstream Stylo now exposes the required basic paint set under Servo. The published 0.19.0 release predates the relevant merge and therefore does not contain it. Official upstream servo/stylo#383 enables fill, fill-opacity, fill-rule, stroke, stroke-width, stroke-linecap, stroke-linejoin, stroke-dasharray, stroke-dashoffset, stroke-miterlimit, and stroke-opacity for Servo. The tested immutable official-upstream revision is that exact merge, a64923b5d5c67313c81c5056f5e30ec0babb04d6. At that revision, 24 longhands in Stylo's SVG style structs remain Gecko-only: -moz-context-properties, clip-rule, color-interpolation, color-interpolation-filters, cx, cy, d, flood-color, flood-opacity, lighting-color, marker-start, marker-mid, marker-end, paint-order, r, rx, ry, shape-rendering, stop-color, stop-opacity, text-anchor, vector-effect, x, and y. Later official upstream servo/stylo#427 enables the eight geometry properties, but geometry is not load-bearing for D-L and is deliberately outside this minimal paint pin. This proves official paint dependency availability, not production ingress or rendered capability.

The options

A refinement of option 3: registered custom properties (@property with a syntax, e.g. <color>) could recover typed computed values for the simple color case — but not paint-server (url(#…)) or context-dependent paint semantics, and servo @property support here is itself unverified. It narrows the loss, it does not remove it, and it needs its own spike.

Decision and scope

The owner took D-L on 2026-07-23:

SVG paint enters the shared cascade through Servo-capable support maintained in official upstream Stylo. Prefer the first published release containing servo/stylo#383; until one exists, use the tested immutable official-upstream revision a64923b5d5c67313c81c5056f5e30ec0babb04d6. Do not use a floating upstream branch or a private source fork.

This decides dependency provenance only. It does not claim that the production Web path admits SVG presentation attributes at presentation-hint precedence, admits SVG-namespace stylesheets, has a conforming SVG/XML entry, consumes the newly available computed values, supports the 24 remaining Gecko-only SVG longhands, or passes an SVG-vector capability gate. E4–E5 remain evidence that the missing ingress dimensions are technically closeable; E7 removes the need to own the basic-paint patch privately. The proving shell's direct-attribute fallback remains deliberately narrow until those separate capability steps are implemented and gated.