The CSS Selectors specification defines the :root pseudo-class that matches root nodes:
https://www.w3.org/TR/selectors-4/#the-root-pseudo

JavaFX uses the non-standard .root style class for the same purpose. We should also support the :root pseudo-class to increase the compatibility of JavaFX CSS with the web specification.

Additionally, we should also support the following child-indexed pseudo-classes:
:first-child
:last-child
:only-child
:nth-child() with arguments "even" and "odd"