Nodes can currently have multiple parents. Under the no-allocation settings though, the number of parents is limited to one, as reported by @mcrvz in #176.

This creates divergence, but the correct behavior is actually to limit the number of parents to 1 across all platforms (likely by not storing a vec). Multiple parents cannot be defined in HTML, and there is no logical interpration of how style inheritance should work with more than one parent.

This should both improve robustness and performance.