There is really no need to use CSS classes here and in most rules below. Element IDs would do just fine. There is a potential speed benefit when using IDs, at least via writing a specialized querySelector which redirects to getElementById for pure IDs. Even if/when that's not the case anymore, I think ID is semantically more informative here: it tells me right away that there is just one `#outline`, not undefined number of `.outline`s. Etc.