SVG path animation: unclear behavior for from + by when commands mismatch

[viralipurbey]

There is an interoperability gap in the SVG spec 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.