EEBO:
Gracefully degrading characters

letter? y     roman?    y     diacr?      y    macr/acute   y  vowel?    y   use ISO
                                                                         n   use ABBR
                                                            n  sym?      y   use entity
                                                                         n   use ABBR
                                                       
                                          n    letter only


                        n     word?       y    
                                          n    familiar?    y  use ISO
                                                            n  use GAP or UNCLEAR


        n     sym?      y     use entity
                        n     legible?    y   familiar?     y  use ISO
                                                            n  use GAP
                                          n   use UNCLEAR