In https://drafts.csswg.org/css-display-3/#valdef-display-contents, there's no explicit mention to how display contents should behave when specified on the root element, i.e., html { display: contents }.

There are a few places in the CSS spec where some elements (like the root element, out of flow positioned elements, floats, or child of grid/flex items) get "blockified". It makes sense to allow display: contents on most of these elements, since there are use cases for display: contents under a flex or grid element.

The only shipping implementation so far (Gecko's) allows specifying display: contents on out of flow positioned elements and floats, but not the root. The in-progress blink implementation has aligned with Gecko in this regard, but this should be specified somewhere, in order to prevent compat issues.

cc @rune-opera