For example, you might expect this to define a fancy animation, ignoring the plain one:

@layer plain fancy;

@layer fancy {
  @keyframes anim { /* ... fancy animation ... */ }
}

@layer plain {
  @keyframes anim { /* ... plain animation ... */ }
}

It might be weird if it defines a plain animation? So probably another reasonable behavior is to make name-defining things
(e.g. @keyframes, @property, @scroll-timeline, etc) parse errors if they appear inside @layer.

cc @mirisuzanne @xiaochengh