[svgwg] Issue: SVG path animation: unclear behavior for from + by when commands mismatch (#1056) marked as Agenda+

viralipurbey has just labeled an issue for https://github.com/w3c/svgwg as "Agenda+":

== SVG path animation: unclear behavior for from + by when commands mismatch ==
There is an interoperability gap in the [SVG spec](https://www.w3.org/TR/SVG2/paths.html#TheDProperty) for animate on path (attributeName="d") when using from + by and the path commands are incompatible.

````
<svg width="200" height="200" xmlns="http://www.w3.org/2000/svg">
   <path id="animated" d="" fill="green">
    <animate attributeName="d"
             from="M10,10 L50,10 L50,50 Z"
             by="M20,20 C40,20 40,40 60,40 Z"
             begin="0s"
             dur="2s"
             fill="freeze" />
  </path>
</svg>
````

**Observed behavior (Gecko)**
The path is not rendered during the animation.

**Spec ambiguity**
The spec says incompatible values fall back to discrete animation, but it does not define:

- what value should be rendered (from, base value, or none), or
- whether rendering nothing is conforming behavior.

For path animations, incompatible command structures cannot be meaningfully added or interpolated, making discrete fallback unclear for from/by and additive animations.

**Request**
Clarify in the SVG spec the expected behavior for path animations with incompatible commands:

- Is rendering nothing valid?
- Should discrete fallback use a specific value?
- How should from/by additive animations be handled?

This clarification would improve interoperability and reflect real engine constraints.


See https://github.com/w3c/svgwg/issues/1056


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Received on Tuesday, 3 February 2026 07:15:47 UTC