Can't find absolute positioning rules in CSS specification

Question

According to Mozilla Developer Network's article on CSS position property, setting position as absolute has the following behavior:

absolute

Do not leave space for the element. Instead, position it at a specified position relative to its closest positioned ancestor if any, or otherwise relative to the initial containing block. Absolutely positioned boxes can have margins, and they do not collapse with any other margins.

However, in the official CSS specification as of 2015 I cannot find where does it states the constraint that the containing box or closest ancestor needs to be positioned (i.e. has to have a computed positioned property of either relative, absolute, fixed or sticky). How is it deduced or implied?

Answer 1

It's in the spec (you were looking at the wrong document):

9.8.4 Absolute positioning

The containing block for a positioned box is established by the nearest positioned ancestor (or, if none exists, the initial containing block).

This is relevant, as well:

9.3.2 Box offsets: top, right, bottom, left

An element is said to be positioned if its position property has a value other than static.

Answer 2

From the official CSS specification as of 2015 is:

This document collects together into one definition all the specs that together form the current state of Cascading Style Sheets (CSS) as of 2015. The primary audience is CSS implementers, not CSS authors, as this definition includes modules by specification stability, not Web browser adoption rate.

Inside the above link you can find a link to the CSS2, where inside you can find this:

9.6 Absolute positioning

In the absolute positioning model, a box is explicitly offset with respect to its containing block. It is removed from the normal flow entirely (it has no impact on later siblings). An absolutely positioned box establishes a new containing block for normal flow children and absolutely (but not fixed) positioned descendants. However, the contents of an absolutely positioned element do not flow around any other boxes. They may obscure the contents of another box (or be obscured themselves), depending on the stack levels of the overlapping boxes.