The specific spec text that matters is here (for flexbox): >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.) https://drafts.csswg.org/css-flexbox-1/#ref-for-propdef-display%E2%91%A5 And the similar text for grid: https://drafts.csswg.org/css-grid-1/#ref-for-propdef-display%E2%91%A4
Bug 1562039 Comment 2 Edit History
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The specific spec text that matters is here (for flexbox): >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.) https://drafts.csswg.org/css-flexbox-1/#ref-for-propdef-display%E2%91%A5 And the similar text for grid: https://drafts.csswg.org/css-grid-1/#ref-for-propdef-display%E2%91%A4