Allow HTML tags, CSS combinators, multiple CSS classes in Fixed Prefix Value

Problem/Motivation

I basically couldn't even target the node body field without also applying the CSS to the body of the comments under it, with a fresh install of standard Drupal 10.3.5!

For that (and to make this module much more useful imho) we would need to put one of these in the Fixed Prefix Value at the Manage display > Format settings (supposing the user wrote a simple p { color: red; } CSS):

1. HTML Tag + CSS Combinators, like: article > .node__content
2. Multiple CSS Classes, like: .node__content .field--name-body

..but either way we would currently get Error message: The Fixed Prefix Value must be a valid CSS class.

Proposed resolution

Remaining tasks

User interface changes

Data model changes