Pablo Rodriguez schrieb am 02.09.2023 um 12:35:
> [...]
> Sorry, but I don‘t know why this doesn‘t get the slanted font as bold (I
> know I should know better, but I‘m stuck at this):
>
>    \definefontfamily[mainface][rm][DejaVu Sans]
>     [sl≃style:bf]

You're using the wrong symbol (you have a math operator in your example) 
to assign a value to a key.

Wolfgang