° 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>ø</sup> (example: Cø).
<sup>△</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.