Incorrect scoping in keyframes at-rule

[yhatt]

@keyframes at-rule in theme CSS is not working by the incorrect element scoping.

/* @theme example */

@keyframes spin {
  from { transform: rotate(0deg); }
  to { transform: rotate(360deg); }
}

Marpit v0.3.1 will convert into:

/* @theme example */

@keyframes spin {
  div.marpit > section from { transform: rotate(0deg); }
  div.marpit > section to { transform: rotate(360deg); }
}

Marpit is misrecognized from, to, and xxx% as the element.