Right now https://drafts.csswg.org/css-grid/#grid-item-display says:

The display value of a grid item is blockified: if the specified display of an in-flow child of an element generating a grid container is an inline-level value, it computes to its block-level equivalent. (See CSS2.1§9.7 [CSS21] and CSS Display [CSS3-DISPLAY] for details on this type of display value conversion.)

Similarly https://drafts.csswg.org/css-flexbox/#flex-items says:

The display value of a flex item is blockified: if the specified display of an in-flow child of an element generating a flex container is an inline-level value, it computes to its block-level equivalent. (See CSS2.1§9.7 [CSS21] and CSS Display [CSS3-DISPLAY] for details on this type of display value conversion.)

As @Loirooriol pointed out in https://bugzilla.mozilla.org/show_bug.cgi?id=1562039, this doesn't match browsers, and browsers use the computed display value of the closest non-display: contents ancestor, even if the element doesn't become a flex or grid container (due to it being replaced for example, or it being in a display: none subtree).

I think the spec in this case should match browsers in this case, in order to not make style of children depend on layout box construction. This also matches how the CSS 2 fixups work. The root element is blockified even if it's a replaced element or wouldn't generate an inline-level box otherwise.