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. https://drafts.csswg.org/css-position/#stacking > The root element always forms a stacking context regardless. Blink and WebKit are correct, the root element always establishes a stacking context. But Firefox doesn't establish a stacking context for `z-index: auto` with `position: relative` or `position: absolute`.
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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. https://drafts.csswg.org/css-position/#stacking > The root element always forms a stacking context regardless. Blink and WebKit are correct, the root element always establishes a stacking context. But Firefox doesn't establish a stacking context for `z-index: auto` with `position: relative` or `position: absolute`.