[css-position] Does the root element establish a stacking context?

[Loirooriol]

https://www.w3.org/TR/CSS22/visuren.html#propdef-z-index defines z-index: auto with

If the box has position: fixed or if it is the root, it also establishes a new stacking context.

Similarly, https://drafts.csswg.org/css2/#propdef-z-index says

The box does not establish a new stacking context unless it is the root element.

But https://drafts.csswg.org/css-position/#stacking doesn't mention anything like that.

Testcase:

:root { box-shadow: inset 0 0 0 50vmax orange; height: 100%; /*position: relative;*/ /*z-index: 0*/ }
:root::before { content: "Text"; position: relative; z-index: -1 }

Firefox: the root element establishes a stacking context if not positioned or if z-index is not auto.
Blink/WebKit: the root element always establishes a stacking context.

[fantasai]

Added a note that the root element always forms a stacking context. (The formal definition is left to CSS2, since we haven't ported those sections over yet.) Lmk if that works for you.

[Loirooriol]

Yeah I guess that works for now.

[tabatkins]

https://w3c.github.io/csswg-drafts/css-position-4/#paint-a-document also now explicitly paints the root element as a stacking context.