Finding: SVG paint in the shared cascade

Genre: finding — grounded evidence for an open owner decision. Not a spec and not a plan. It records what was established while building the Web-first prototype, so the decision it gates can be taken on evidence rather than assumption.

Status: open. Names a decision to file in the charter's registry. No option below is chosen here.

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. SVG paint is not among them, so today the SVG semantic compiler reads paint from presentation attributes and inline styles outside the cascade. Closing that gap is a real cost with no free option; this finding lays the options out.

Evidence

• E1 — the entire SVG paint/geometry property set is 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 is not a viable build here. 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 buildable in this pure-Rust Cargo workspace.

• E4 — the SVG value types themselves compile under servo. The generic, specified, and computed svg value modules (e.g. the generic SVG-paint type) are compiled unconditionally, not gecko-gated. Only the property declarations are engine-gated. A fork that un-gated the SVG paint longhands for servo is therefore plausible — but unverified at build depth, and it means carrying a patch to a large, fast-moving dependency.

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.

Recommendation (for the owner to decide)

No option is free, so this is a genuine registry decision. The evidence suggests:

• Keep option 4 as the near-term posture. It is correct for the paint that the prototype's fixtures actually use and is honest about what it does not do.
• Do not adopt option 3 as the cascade of record. A carrier that reproduces cascade mechanics while discarding SVG paint semantics is, in spirit, another temporary shim — the same thing the amendment rules out when it forbids promoting the temporary SVG-only matcher. It may have a place as a narrow bridge for the cascaded-rule case, but only behind an explicit decision, never as the default.
• Scope option 2 as the path most faithful to "one browser-grade cascade." It is the only option that yields real SVG paint computed values without Gecko. Before committing to it, a timeboxed fork-feasibility spike should confirm the servo build actually computes the un-gated longhands — E4 makes it plausible, not proven.

The decision to file

File in the charter's decision registry: how SVG paint enters the shared cascade — status quo (attribute read) vs a Stylo fork vs a scoped custom-property bridge. Its evidence bar is this finding plus, if option 2 is pursued, the fork-feasibility spike. Until it is decided, the Web-first path reads SVG paint outside the cascade and says so.