The New Yorker has a famously idiosyncratic style guide that calls for diaresis to be explicitly marked (in words like e.g. coöperate). This is pretty interesti

The New Yorker has a famously idiosyncratic style guide that calls for diaresis to be explicitly marked (in words like e.g. coöperate). This is pretty interesting though because it seems to indicate that the pronunciation has shifted - nobody would pronounce "aero" with a diaresis today.