I noticed that few display typefaces support the
IPA. Why bother? Well, there are linguists and teachers active on Instagram, so the
display fonts used there to decorate the stories would be much sweeter with IPA!
The few display fonts I could find that contained the characters in the IPA range, failed to also include the Greek letters that are part of the IPA: beta (β), theta (θ), chi (χ).
Sidenote: I read somewhere that an IPA-specific chi should be used in case the font's default Greek chi shares its design with /x. It took me quite a few minutes to flick the thick covers of dust from this obscure character: it's Latin small letter chi U+AB53. I wonder how many input tools use it instead of the Greek chi. This one
doesn't. For fonts that support also Greek and feature a non-descending chi, maybe a contextual substitution could be implemented?*
It is maybe also worth noting that Latin small letter gamma doesn't necessarily share the design with Greek gamma: I suspect the loop should be always present in the Latin one. I wonder about the latin phi: it usually features horizontal UC-style serifs, which I find visually disturbing. Is this necessary? Isn't using the asc-desc form instead of the cursive desc-only form enough differentiation?
Of lesser bugs, small caps fonts in the families I investigated sometimes contained small caps versions of component characters (ʍ, ʌ) but not manually drawn ones (ɪ). Small caps don't make any sense for IPA, except for the IPA letters used for some African orthographies (the ones with the hook: ɓɗɠɦŋʋ, then Latin gamma: ɣ and some more: ɛɔ). What is the solution?
Perhaps stop creating SC cuts and provide OpenType small caps instead. But if you insisted... Maybe stretch the non-capitalizable IPA letters up to the SC height, if different than x-height? But if we consider the OT solution, again, what if someone applies SC to an IPA string containing a letter used for African that does have a SC variant? Maybe inhibit the substitution if a non-African IPA letter is found in the same word (that would be a lot of long lines in a .fea file and the word length would be limited, but a simplistic solution could just check the letter before and after). Or conversely, if SC IPA is designed, provide two versions of SC African IPA letters, one based on the UC, and one on the LC.
All this agonizing is of course largely theoretical and futile. But then again, indirectly quoting from somewhere on this forum, what the font designer deems shouldn't be done, the graphic designer will do, or something along these lines.
Bottomline: don't forget to provide your font with beta, theta, and chi from the Greek and Coptic range if you plan to support IPA. Possibly include the Latin chi U+AB53 from the abyss-of-Unicode Latin Extended-E range.
* Sometimes I feel like I'm against meddling with character substitution like Romanian comma accents. It is not the font's responsibility to correct the user's typing errors or the layout software manufacturer's error... But.