When styling elements in Shadow DOM, combinators can be used with the :host selector to select elements in the shadowRoot (e.g. :host > div { ... }). These rules do not currently work with CSS nesting.

It's not clear if this is a bug or omission or intentional (hope not) via the language at https://drafts.csswg.org/css-nesting/#nesting.

Note that the behavior of combinators next to :host is somewhat unique in that the tree traversed is the shadow tree, not the tree in the scope of the host element. This likely(?) means this needs a specific callout in the spec.