@radames Thanks!
Technically `:scope` doesn't do much on its own because no browser implements scoped stylesheets - it's equivalent to `:root` and `html`. However, it's also still a valid selector, so I'm using it as a placeholder which I then replace with the actual selector in addCss() (https://observablehq.com/@mootari/eases-on-edge#addCss).
I've used this technique a few times now (e.g. in https://observablehq.com/@mootari/range-slider) to make the authoring of CSS themes easier.