@walpolea Visually speaking, it'd be great if you could use (sub)grid to align items in your `.track-definitions`, not critical, just nicer looking.

And the "Add" input below should react to the Enter key if possible.

In the code, I love the indentation to show the track hierarchy! A detail, though `:where(*)` is the same as `*` so you can shave a little cognitive overhead there, but I can understand wanting to keep that for customisation. And a copy to Clipboard would be a nice addition.