While ♯, ♭ and ♮ do work in post bodies, they don't work in comments and in question titles. But you can copy the Unicode characters from other posts and use them instead; all computer systems (including my old Moto E) render these correctly nowadays.

° is the named HTML entity for the (temperature or arc) degree symbol °, which according to Wikipedia doubles as symbol for diminished chords.

For half-diminished chords you could use <sup>&oslash;</sup> (example: C^ø).

<sup>&#9651;</sup> (example: C^△) produces a symbol which can be used for a major seventh chord.

While ♯, ♭ and ♮ do work in post bodies, they don't work in comments and in question titles. But you can copy the Unicode characters from other posts and use them instead; all computer systems (including my old Moto E) render these correctly nowadays.