One subtle thing worth adding is that display: contents also changes how accessibility trees are constructed. The element is removed from the visual layout and from the accessibility tree in many browsers, so semantics like list grouping, landmarks, or ARIA roles can disappear unless you re-introduce them manually.

That’s why subgrid ends up filling a different niche: you preserve the DOM structure, preserve accessibility semantics, and still let the children participate in the parent’s track sizing. It costs more than contents, but it avoids a lot of the accidental side-effects that show up once you start mixing layout, semantics, and interactivity.

Yeah this is a good callout. My understanding is that display: contents is not meant to impact the accessibility tree but there is a long and ongoing history of browser bugs that make me not want to use it for elements that have an accessible role