As specified today, shadow DOM subtrees only replace contents of a shadow host. Jonas mentioned that we've once talked about enabling to also not render the shadow host itself. A simple use case here would be using shadow DOM subtrees to build more than one row of a table.

One way this could be done is by specifying "display:transparent", which informs the rendering that the DOM element it matches is removed from rendering and substituted with its children.

The path here would be to specify "display:transparent", then specify shadow host and shadow root rendering in a way where by default, shadow root has "display:transparent" applied, and shadow host does not. Then the authors could simply set shadow host to "display:transparent" and eat their cakes.