A node that can be passed to stylo and selectors that allows accessing the
parent node. We consider this to be too dangerous for normal layout, so it is
reserved only for using stylo and selectors.
A trait that exposes a DOM nodes to layout. Implementors of this trait must abide by certain
safety requirements. Layout will only ever access and mutate each node from a single thread
at a time, though children may be used in parallel from other threads. That is why this trait
does not allow access to parent nodes, as it would make it easy to cause race conditions and
memory errors.