In section 9.9.1. Specifying the stack level: the z-index property, in the description of the Example 152 there is this sentence:

The "text2" stack level is inherited from the root box.

This sentence is a bit misleading in my opinion. z-index property is not inherited property. In the example "text2" doesn't have z-index specified, hence its computed z-index value is auto. The value of auto means that the stack level of the generated box in the current stacking context is 0. This is in my opinion more accurate explanation of where the value of stack level of this element comes from. It's not inherited from the root box.