Comment by cryptonector

> but only as a y with a diæresis

á (á) is also an 'a' with a diacritical acute accent. If you mean that ÿ should not have a precomposition in Unicode, well, why not, especially if it really is used in "a handful of proper nouns in French and Hungarian".

Remember, the reason we have combining marks is that that is in fact how many of these characters were composed in actual use, especially with typewriters. Heck, 1966 US-ASCII / ECMA-6 (1985), section 5, describes the use of backspace/overstrike in US-ASCII for composition of such characters! That comes from typewriter use. And that's where compose-key sequences generally come from, too.

So it's not at all surprising that given that ÿ has _some_ use, therefore a) it is a valid glyph to construct with combining diaeresis, and b) that it has a precomposed codepoint in Unicode.