def vtt_talk_sources(ttFont) -> List[str]:
  """Return the tags of VTT source tables found in a font."""
  VTT_SOURCE_TABLES = {'TSI0', 'TSI1', 'TSI2', 'TSI3', 'TSI5'}
  tables_found = [tag for tag in ttFont.keys() if tag in VTT_SOURCE_TABLES]
  return tables_found
Return the tags of VTT source tables found in a font.