When an element is invalidated, it's possible for any subsequent sibling
or any of their descendants to also need invalidation. (Due to the CSS
sibling combinators, `+` and `~`)
For DOM node insertion/removal, we must also invalidate preceding
siblings, since they could be affected by :first-child, :last-child or
:nth-child() selectors.
This increases the amount of invalidation we do, but it's more correct.
In the future, we will implement optimizations that drastically reduce
the number of elements invalidated.
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