The "perspective" property is specified to only allow non-negative values: https://www.w3.org/TR/css-transforms-2/#perspective-property. However, we don't clip values that would be computed to less than 0, for instance using a calc() expression, and instead deem them invalid internally and thus consider as if "none" was provided, which has implications when blending as blending with "none" means a discrete behavior should be used.