[css-forms-1] Define which structural pseudo elements work for base appearance inputs.

[lukewarlow]

To enable true interopability with styling we need to specify which pseudo elements work with inputs.

Currently ::before and ::after only work on file inputs and even that isn't the case in Firefox (WebKit also renders ::after such that it overlaps the file label).

Also see #11928 (comment) regarding ::first-line and ::first-letter on input buttons.