contain:layout and contain:size are specced as making an element "[establish] an independent formatting context" https://drafts.csswg.org/css-contain/#containment-layout https://drafts.csswg.org/css-contain/#containment-paint (For blocks, this prevents margin-collapsing and float interactions across the containment boundary.) For grid: if this is set on an element which is a subgrid, it needs to make that element *not* be a subgrid. (It can't participate in its parent's grid formatting context.) (There might be other things that make an element "establish an independent formatting context" that we need to consider for subgrid-disabling, too...? not sure)
Bug 1471758 Comment 0 Edit History
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contain:layout and contain:paint are specced as making an element "[establish] an independent formatting context" https://drafts.csswg.org/css-contain/#containment-layout https://drafts.csswg.org/css-contain/#containment-paint (For blocks, this prevents margin-collapsing and float interactions across the containment boundary.) For grid: if this is set on an element which is a subgrid, it needs to make that element *not* be a subgrid. (It can't participate in its parent's grid formatting context.) (There might be other things that make an element "establish an independent formatting context" that we need to consider for subgrid-disabling, too...? not sure)