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Doesn't the definition of vertexElementRelativeOffset still allow for negative vertexElementIndex? E.g., consider

|vertexElementIndex| * |vertexBufferLayout|.{{GPUVertexBufferLayout/arrayStride}} +
                |attributeDesc|.{{GPUVertexAttribute/offset}}.

as long as |attributeDesc|.{{GPUVertexAttribute/offset}} >= -|vertexElementIndex| * |vertexBufferLayout|.{{GPUVertexBufferLayout/arrayStride}} then as written I believe vertexElementIndex < 0 would still be allowed.

Is negative vertexElementIndex supported across all APIs? I could only find reference in the GL specification which says it is not. Specifically I would wonder which values are populated in vertex_index on the shader side.