[css-pseudo-4] Layering of highlight pseudo backgrounds

Currently the spec specifies that e.g. if ::selection and ::spelling-error and ::grammar-error all apply to the same piece of text and each specifies a background color, these are layered, which will be noticeable if any of them are semi-transparent.

Just want to confirm that this is the model we want. (The alternative would be that only the topmost color paints, unless it is the keyword transparent or something.)