This implementation currently implements the el-Upper transform from CLDR.

CLDR algorithm

According to CLDR all accents on all characters are are omitted when upcasing.

Remove 0301 following Greek, with possible intervening 0308 marks. ::NFD(); For uppercasing (not titlecasing!) remove all greek accents from greek letters. This is done in two groups, to account for canonical ordering. [:Greek:] [^[:ccc=Not_Reordered:][:ccc=Above:]]? { [̓̔́̀̆͂̈̄] → ; [:Greek:] [^[:ccc=Not_Reordered:][:ccc=Iota_Subscript:]]? { ͅ → ; ::NFC();

That transform basically says remove all accents except a subscripted iota. It doesn't handle dipthongs correctly.

Mozilla algorithm

Mozilla has a thread on a bug report that:

Greek accented letters should be converted to the respective non-accented uppercase letters. The required conversions are the following (in Unicode):

ά -> Α έ -> Ε ή -> Η ί -> Ι ΐ -> Ϊ ό -> Ο ύ -> Υ ΰ -> Ϋ ώ -> Ω

Also diphthongs (two-vowel constructs) should be converted as follows, when the first vowel is accented:

άι -> ΑΪ έι -> ΕΪ όι -> ΟΪ ύι -> ΥΪ άυ -> ΑΫ έυ -> ΕΫ ήυ -> ΗΫ όυ -> ΟΫ

That thread seems to align with current-day Mozilla which says the rules are:

In Greek (el), vowels lose their accent when the whole word is in uppercase (ά/Α), except for the disjunctive eta (ή/Ή). Also, diphthongs with an accent on the first vowel lose the accent and gain a diaeresis on the second vowel (άι/ΑΪ).